Email non-delivery report (NDR) and SMTP errors in Exchange Online – Exchange

432 4.3.2 STOREDRV.Deliver; recipient thread limit exceeded The recipient mailbox’s ability to accept messages is being throttled because it’s receiving too many messages too quickly. This is done so a single recipient’s mail processing doesn’t unfairly impact other recipients sharing the same mailbox database. For more information about this by-design throttling, see Store Driver Fault Isolation Improvements in Exchange 2010 SP1. 4.4.316 Connection refused [Message=Socket error code 10061] Microsoft 365 or Office 365 is trying to send a message to an email server outside of Microsoft 365 or Office 365, but attempts to connect to it are failing due to a network connection issue at the external server’s location. This error almost always indicates an issue with the receiving server or network outside of Microsoft 365 or Office 365. The error should also include the IP address of the server or service that’s generating the error, which you can use to identify the party responsible for fixing this. 4.4.7 Message expired The message in the queue has expired. The sending server tried to relay or deliver the message, but the action wasn’t completed before the message expiration time occurred. This message can also indicate that a message header limit has been reached on a remote server, or some other protocol time-out occurred while communicating with the remote server. This message usually indicates an issue on the receiving server. Check the validity of the recipient address, and determine if the receiving server is configured correctly to receive messages.

You might have to reduce the number of recipients in the message header for the host about which you’re receiving this error. If you send the message again, it’s placed in the queue again. If the receiving server is available, the message is delivered.

For more information, see Fix email delivery issues for error code 4.4.7 in Exchange Online.

4.4.8 MX hosts of  failed MTA-STS validation The destination MX host was not the host expected per the domain’s STS policy. This error usually indicates an issue with the destination domain’s MTA-STS policy not containing the MX host. For more information, see Enhancing mail flow with MTA-STS. 4.5.3 Too many recipients The message has more than 200 SMTP envelope recipients from the same domain. An envelope recipient is the original, unexpanded recipient that’s used in the RCPT TO command to transmit the message between SMTP servers. When this error is returned by Microsoft 365 or Office 365, the sending server must break up the number of envelope recipients into smaller chunks (chunking) and resend the message. 4.7.5 Remote certificate failed MTA-STS validation. Reason:  The destination mail server’s certificate must chain to a trusted root Certificate Authority and the Common Name or Subject Alternative Name must contain an entry for the host name in the STS policy. This error usually indicates an issue with the destination mail server’s certificate. For more information, see Enhancing mail flow with MTA-STS. 4.7.26 Access denied, a message sent over IPv6 [2a01:111:f200:2004::240] must pass either SPF or DKIM validation, this message is not signed The sending message sent over IPv6 must pass either SPF or DKIM. For more information, see Support for anonymous inbound email messages over IPv6. 4.7.321 starttls-not-supported: Destination mail server must support TLS to receive mail.

• DNSSEC checks have passed, yet upon connection, destination mail server doesn’t respond to the STARTTLS command.
• The destination server responds to the STARTTLS command, but the TLS handshake fails.

This message usually indicates an issue on the destination email server. Check the validity of the recipient address. Determine if the destination server is configured correctly to receive the messages. 4.7.322 certificate-expired: Destination mail server's certificate is expired. DNSSEC checks have passed, yet upon establishing the connection, the destination mail server provides a certificate that’s expired. A valid X.509 certificate that isn’t expired must be presented. X.509 certificates must be renewed after their expiration, most commonly on an annual basis. 4.7.323 tlsa-invalid: The domain failed DANE validation. Records are DNSSEC authentic, but one or multiple of these scenarios occurred:

• The destination mail server’s certificate doesn’t match with what’s expected per the authentic TLSA record.
• Authentic TLSA record is misconfigured.
• Destination domain is being attacked.
• Any other DANE failure.

This message usually indicates an issue on the destination email server. Check the validity of the recipient address and determine if the destination server is configured correctly to receive messages. For more information, see DANE protocol: updates and operational guidance. 4.7.324 dnssec-invalid: Destination domain returned invalid DNSSEC records The destination domain indicated it was DNSSEC-authentic, but Exchange Online wasn’t able to verify it as DNSSEC-authentic. For more information, see Overview of DNSSEC. 4.7.325 certificate-host-mismatch: Remote certificate MUST have a common name or subject alternative name matching the hostname (DANE) This happens when the presented certificate identities (CN and SAN) of a destination SMTP target host don’t match any of the domains or MX host. This message usually indicates an issue on the destination email server. Check the validity of the recipient address and determine if the destination server is configured correctly to receive messages. For more information, see How SMTP DNS-based Authentication of Named Entities (DANE) works to secure email communications. 4.7.500-699 Access denied, please try again later Suspicious activity has been detected and sending has been temporarily restricted for further evaluation. If this activity is valid, this restriction will be lifted shortly. 4.7.850-899 Access denied, please try again later Suspicious activity has been detected on the IP in question, and it has been temporarily restricted while it’s being further evaluated. If this activity is valid, this restriction will be lifted shortly. 5.0.350 Generic error, x-dg-ref header is too long, or Requested action not taken: policy violation detected (AS345) 5.0.350 is a generic catch-all error code for a wide variety of nonspecific errors from the recipient’s email organization. The specific x-dg-ref header is too long message is related to Rich Text formatted messages. The specific Requested action not taken: policy violation detected (AS345) message is related to nested attachments. For more information, see Fix email delivery issues for error code 550 5.0.350 in Exchange Online. 5.1.0 Sender denied A common cause of this NDR is when you use Microsoft Outlook to save an email message as a file, and then someone opened the message offline and replied to it. The message property only preserves the legacyExchangeDN attribute when Outlook delivers the message, and therefore the lookup could fail. Either the recipient address is incorrectly formatted, or the recipient couldn’t be correctly resolved. The first step in resolving this error is to check the recipient address, and send the message again.

For more information, see Fix email delivery issues for error code 5.1.0 in Exchange Online.

5.1.1 Bad destination mailbox address This failure might be caused by the following conditions:

• The recipient’s email address was entered incorrectly by the sender.
• The recipient’s email address doesn’t exist in the destination email system.
• The recipient’s mailbox has been moved and the Outlook recipient cache on the sender’s computer hasn’t updated.
• An invalid legacy domain name (DN) exists for the recipient’s mailbox Active Directory Domain Service.

This error typically occurs when the sender of the message enters an incorrect email address of the recipient. The sender should check the recipient’s email address and send again. This error can also occur if the recipient email address was correct in the past but has changed or has been removed from the destination email system.

If the sender of the message is in the same organization as the recipient, and the recipient’s mailbox still exists, determine whether the recipient’s mailbox has been relocated to a new email server. If so, Outlook might not have updated the recipient cache correctly. Instruct the sender to remove the recipient’s address from sender’s Outlook recipient cache and then create a new message. Resending the original message will result in the same failure.

For more information, see Fix email delivery issues for error code 5.1.1 through 5.1.20 in Exchange Online.

5.1.8 Access denied, bad outbound sender The account has been blocked for sending too much spam. Typically, this problem occurs because the account has been compromised (hacked) by phishing or malware. For more information, see Fix email delivery issues for error code 5.1.8 in Exchange Online. 5.1.10 Recipient not found The recipient’s wasn’t found by SMTP address lookup. For more information, see Fix email delivery issues for error code 550 5.1.10 in Exchange Online. 5.1.90 Your message can't be sent because you've reached your daily limit for message recipients The sender has exceeded the recipient rate limit as described in Sending limits. This could indicate the account has been compromised and is being used to send spam. For more information, see How to determine whether your account has been compromised. 5.2.2 Submission quota exceeded The sender has exceeded the recipient rate limit or the message rate limit as described in Sending limits. This could indicate the account has been compromised and is being used to send spam. For more information, see How to determine whether your account has been compromised. 5.2.121 Recipient's per hour message receive limit from specific sender exceeded The sender has exceeded the maximum number of messages they’re allowed to send per hour to a specific recipient in Exchange Online. The automated mailer or sender should try again later, and reduce the number of messages they send per hour to a specific recipient.

This limit helps protect Microsoft 365 or Office 365 users from rapidly filling their inboxes with a large number of messages from errant automated notification systems or other single-sender mail storms.

5.2.122 Recipient's per hour message receive limit exceeded The Microsoft 365 or Office 365 recipient has exceeded the number of messages they can receive per hour from all senders. The automated mailer or sender should try again later, and reduce the number of messages they send per hour to a specific recipient.

This limit helps protect Microsoft 365 and Office 365 users from rapidly filling their inboxes with a large number of messages from errant automated notification systems or other mail storms.

5.3.190 Journaling on-premises messages to Microsoft 365 or Office 365 not supported when Journaling Archive is disabled Journaling on-premises messages to Microsoft 365 or Office 365 isn’t supported for this organization because they haven’t turned on Journaling Archive in their settings. A journaling rule is configured in the organization’s on-premises environment to journal on-premises messages to Microsoft 365 or Office 365, but Journaling Archive is disabled. For this scenario to work, the organization’s Office 365 administrator should either enable Journaling Archive or change the journaling rule to journal messages to a different location. 5.4.1 Relay Access Denied The mail server that’s generating the error doesn’t accept mail for the recipient’s domain. This error is caused by mail server or DNS misconfiguration. For more information, see Fix email delivery issues for error code 5.4.1 in Exchange Online. 5.4.1 Recipient address rejected: Access denied The recipient’s address doesn’t exist. For more information, see Use Directory Based Edge Blocking to reject messages sent to invalid recipients. 5.4.6 or 5.4.14 Routing loop detected A configuration error has caused an email loop. 5.4.6 is generated by on-premises Exchange server (you’ll see this code in hybrid environments). 5.4.14 is generated by Exchange Online.

By default, after 20 iterations of an email loop, Exchange interrupts the loop and generates an NDR to the sender of the message.

This error occurs when the delivery of a message generates another message in response. That message then generates a third message, and the process is repeated, creating a loop. To help protect against exhausting system resources, Exchange interrupts the mail loop after 20 iterations. Mail loops are typically created because of a configuration error on the sending mail server, the receiving mail server, or on both. Check the sender’s and the recipient’s mailbox rules configuration to determine whether automatic message forwarding is enabled.

For more information, see Fix email delivery issues for error code 5.4.6 or 5.4.14 in Exchange Online.

5.4.8 MX hosts of  failed MTA-STS validation The destination MX host was not the host expected per the domain’s STS policy. This error usually indicates an issue with the destination domain’s MTA-STS policy not containing the MX host. For more information, see Enhancing mail flow with MTA-STS. 5.4.300 Message expired The email took too long to be successfully delivered, either because the destination server never responded or the sent message generated an NDR error and that NDR couldn’t be delivered to the original sender. 5.5.0 550 5.5.0 Requested action not taken: mailbox unavailable The recipient’s domain is @hotmail.com or @outlook.com and it wasn’t found by SMTP address lookup. Similar to 550 5.1.10. For more information, see Fix email delivery issues for error code 550 5.1.10 in Exchange Online. 5.6.11 Invalid characters Your email program added invalid characters (bare line feed characters) into a message you sent. For more information, see Fix email delivery issues for error code 5.6.11 in Exchange Online. 5.7.1 Delivery not authorized The sender of the message isn’t allowed to send messages to the recipient. This error occurs when the sender tries to send a message to a recipient but the sender isn’t authorized to do this. This error frequently occurs when a sender tries to send messages to a distribution group that has been configured to accept messages only from members of that distribution group or other authorized senders. The sender must request permission to send messages to the recipient.

This error can also occur if an Exchange transport rule rejects a message because the message matched conditions that are configured on the transport rule.

For more information, see Fix email delivery issues for error code 5.7.1 in Exchange Online.

5.7.1 Unable to relay The sending email system isn’t allowed to send a message to an email system that isn’t the final destination of the message. This error occurs when the sending email system tries to send an anonymous message to a receiving email system, and the receiving email system doesn’t accept messages for the domain or domains specified in one or more of the recipients. The following reasons are the most common ones for this error:

• A third party tries to use a receiving email system to send spam, and the receiving email system rejects the attempt. By the nature of spam, the sender’s email address might have been forged, and the resulting NDR could have been sent to the unsuspecting sender’s email address. It’s difficult to avoid this situation.
• An MX record for a domain points to a receiving email system where that domain isn’t accepted. The administrator responsible for the specific domain name must correct the MX record or configure the receiving email system to accept messages sent to that domain, or do both.
• A sending email system or client that should use the receiving email system to relay messages doesn’t have the correct permissions to do this.

For more information, see Fix email delivery issues for error code 5.7.1 in Exchange Online. 5.7.1 Client was not authenticated The sending email system didn’t authenticate with the receiving email system. The receiving email system requires authentication before message submission. This error occurs when the receiving server must be authenticated before message submission, and the sending email system hasn’t authenticated with the receiving email system. The sending email system administrator must configure the sending email system to authenticate with the receiving email system for delivery to be successful.

For more information, see Fix email delivery issues for error code 5.7.1 in Exchange Online.

5.7.5 Remote certificate failed MTA-STS validation. Reason:  The destination mail server’s certificate must chain to a trusted root Certificate Authority and the Common Name or Subject Alternative Name must contain an entry for the host name in the STS policy. This error usually indicates an issue with the destination mail server’s certificate. For more information, see Enhancing mail flow with MTA-STS. 5.7.12 Sender was not authenticated by organization The sender’s message is rejected because the recipient address is set up to reject messages sent from outside its organization. Only an email administrator for the recipient’s organization can change this. For more information, see Fix email delivery issues for error code 5.7.12 in Exchange Online. 5.7.23 The message was rejected because of Sender Policy Framework violation The destination email system uses SPF to validate inbound mail, and there’s a problem with your SPF configuration. For more information, see Fix email delivery issues for error code 5.7.23 in Exchange Online. 5.7.57 Client was not authenticated to send anonymous mail during MAIL FROM You configured an application or device to send (relay) email messages in Microsoft 365 or Office 365 using the smtp.office365.com endpoint, and there’s a problem with the configuration of the application or device. For more information, see Fix email delivery issues for error code 5.7.57 in Exchange Online. 5.7.64 TenantAttribution; Relay Access Denied You use an inbound connector to receive messages from your on-premises email environment, and something has changed in your on-premises environment that makes the inbound connector’s configuration incorrect. For more information, see Fix email delivery issues for error code 5.7.64 in Exchange Online. 5.7.124 Sender not in allowed-senders list The sender doesn’t have permission to send to the distribution group because the sender isn’t in the group’s allowed-senders list. Depending how the group is set up, even the group’s owner might need to be added to the allowed sender list in order to send messages to the group. For more information, see Fix email delivery issues for error code 5.7.124 in Exchange Online. 5.7.133 Sender not authenticated for group The recipient address is a group distribution list that is set up to reject messages sent from outside its organization. Only an email administrator for the recipient’s organization or the group owner can change this. For more information, see Fix email delivery issues for error code 5.7.133 in Exchange Online. 5.7.134 Sender was not authenticated for mailbox The recipient address is a mailbox that is set up to reject messages sent from outside its organization. Only an email administrator for the recipient’s organization can change this. For more information, see Fix email delivery issues for error code 5.7.134 in Exchange Online. 5.7.13 or 135 Sender was not authenticated for public folder The recipient address is a public folder that is set up to reject messages sent from outside its organization. Only an email administrator for the recipient’s organization can change this. For more information, see Fix email delivery issues for error code 5.7.13 or 5.7.135 in Exchange Online. 5.7.136 Sender was not authenticated The recipient address is a mail user that is set up to reject messages sent from outside its organization. Only an email administrator for the recipient’s organization can change this. For more information, see Fix email delivery issues for error code 5.7.136 in Exchange Online. 5.7.25 Access denied, the sending IPv6 address [2a01:111:f200:2004::240] must have a reverse DNS record The sending IPv6 address must have a reverse DNS record to send email over IPv6. For more information, see Support for anonymous inbound email messages over IPv6. 5.7.321 starttls-not-supported: Destination mail server must support TLS to receive mail.

• DNSSEC checks have passed, yet, upon connection, the destination mail server doesn’t respond to the STARTTLS command.
• The destination server responds to the STARTTLS command, but the TLS handshake fails.

This message usually indicates an issue on the destination mail server. Check the validity of the recipient address and determine if the destination server is configured correctly to receive messages. 5.7.322 certificate-expired: Destination mail server's certificate is expired. DNSSEC checks have passed, yet, upon establishing the connection, the destination mail server provides a certificate that is expired. A valid X.509 certificate that isn’t expired must be presented. X.509 certificates must be renewed after their expiration, most commonly on an annual basis. 5.7.323 tlsa-invalid: The domain failed DANE validation. Records are DNSSEC authentic but one or multiple of these things occurred:

• The destination mail server’s certificate doesn’t match with what is expected per the authentic TLSA record.
• Authentic TLSA record is misconfigured.
• Destination domain is being attacked.
• The certificate start date is in the future.
• Any other DANE failure.

This message usually indicates an issue on the destination mail server. Check the validity of the recipient address and determine if the destination server is configured correctly to receive messages.

For more information about DANE, see https://datatracker.ietf.org/doc/html/rfc7671.

5.7.324 dnssec-invalid: Destination domain returned invalid DNSSEC records The destination domain indicated it was DNSSEC authentic but Exchange Online wasn’t able to verify it as DNSSEC authentic. For more information about DNSSEC, see Overview of DNSSEC. 5.7.325 certificate-host-mismatch: Remote certificate MUST have a common name or subject alternative name matching the hostname (DANE) This error occurs when the presented certificate identities (CN and SAN) of a destination SMTP target host don’t match any of the domains or MX host. This message usually indicates an issue on the destination email server. Check the validity of recipient address and determine if the destination server is configured correctly to receive messages. For more information, see How SMTP DNS-based Authentication of Named Entities (DANE) works to secure email communications. 5.7.501 Access denied, spam abuse detected The sending account has been banned due to detected spam activity. For more information, see Fix email delivery issues for error code 451 5.7.500-699 (ASxxx) in Exchange Online.

Verify that account issues have been resolved, and reset its credentials. To restore this account’s ability to send mail, contact support through your regular channel.

5.7.502 Access denied, banned sender The sending account has been banned due to detected spam activity. Verify that account issues have been resolved, and reset its credentials. To restore this account’s ability to send mail, please contact support through your regular channel. 5.7.503 Access denied, banned sender The sending account has been banned due to detected spam activity. Verify that account issues have been resolved, and reset its credentials. To restore this account’s ability to send mail, please contact support through your regular channel. 5.7.504 [[email protected]]: Recipient address rejected: Access denied The recipient address that you’re attempting to contact isn’t valid. Verify the recipient’s email address, and try again. 5.7.505 Access denied, banned recipient The recipient that you’re attempting to contact isn’t valid. If you feel this is an error, contact support. 5.7.506 Access Denied, Bad HELO Your server is attempting to introduce itself (HELO according to RFC 821) as the server it’s trying to connect to, rather than its own fully qualified domain name. This isn’t allowed, and it’s characteristic of typical spambot behavior. 5.7.507 Access denied, rejected by recipient The IP that you’re attempting to send from has been blocked by the recipient’s organization. Contact the recipient to resolve this issue. 5.7.508 Access denied, [$SenderIPAddress] has exceeded permitted limits within $range range The sender’s IPv6 range has attempted to send too many messages in too short a time period. Not applicable 5.7.509 Access denied, sending domain [$SenderDomain] does not pass DMARC verification and has a DMARC policy of reject. The sender’s domain in the 5322.From address doesn’t pass DMARC. For information on why this error occurred, see Why does DMARC fail?.

A user too receives this Bounce Message because it failed DMARC and the DMARC policy is set to reject all failures. The user then should contact their email administrator for additional help.

5.7.510 Access denied, [contoso.com] does not accept email over IPv6 The sender is attempting to transmit a message to the recipient over IPv6, but the recipient doesn’t accept email messages over IPv6. Not applicable 5.7.511 Access denied, banned sender The IP that you’re attempting to send from has been banned. To delist the address, email [email protected] and provide the full NDR code and IP address.

For more information, see Use the delist portal to remove yourself from the blocked senders list.

5.7.512 Access denied, message must be RFC 5322 section 3.6.2 compliant Message was sent without a valid “From” email address. Office 365 only. Each message must contain a valid email address in the “From” header field. Proper formatting of this address includes angle brackets around the email address, for example, . Without an address with this format, Microsoft 365 or Office 365 will reject the message. 5.7.513 Service unavailable, Client host [$ConnectingIP] blocked by $recipientDomain using Customer Block list (AS16012607) The recipient domain has added your sending IP address to its custom blocklist. The domain that received the email has blocked your sender’s IP address. If you think your IP address has been added to the recipient domain’s custom blocklist by error, you need to contact them directly and ask them to remove it from the blocklist. 5.7.606-649 Access denied, banned sending IP [IP1.IP2.IP3.IP4] The IP that you’re attempting to send from has been banned. Verify that you’re following the best practices for email deliverability, and ensure your IPs’ reputations haven’t been degraded as a result of compromise or malicious traffic. If you believe you’re receiving this message by error, you can use the self-service portal to request your IP address to be removed from this list.

For more information, see Use the delist portal to remove yourself from the blocked senders list.

5.7.703 Your message can't be delivered because messages to XXX, YYY are blocked by your organization using Tenant Allow Block List. Someone in your organization sent mail to an email address or domain that’s blocked in the Tenant Allow/Block List. The entire message is blocked for all internal and external recipients of the message, even if only one recipient email address or domain is defined in a block entry. 5.7.705

5.7.708

5.7.705 Access denied, tenant has exceeded threshold, 5.7.708 Access denied, traffic not accepted from this IP Most of the traffic from this tenant has been detected as suspicious and this detection has resulted in a ban on the sending ability for the tenant. Ensure that any compromises or open relays have been resolved, and then contact support through your regular channel.

For more information, see Fix email delivery issues for error codes 5.7.700 through 5.7.750 in Exchange Online.

5.7.750 Service unavailable. Client blocked from sending from unregistered domains A suspicious number of messages from unprovisioned domains is coming from this tenant. Add and validate any or all domains that you use to send email from Microsoft 365 or Office 365.

For more information, see Fix email delivery issues for error codes 5.7.700 through 5.7.750 in Exchange Online.

n/a The message can't be submitted because the sender's submission quota was exceeded The user account has exceeded the recipient rate limit (10,000 recipients per day). The account has likely been compromised. For more information, see Fix email delivery issues for error ‘the sender’s submission quota was exceeded’ in Exchange Online.