i am experiencing this issue when replying emails from outlook.com (through browser) and also via mobile app.
To be precise now, I first noticed this issue when once again was replying to email chain from Cooperative. I had been writing with Cooperative for years and suddenly emails got stuck, so I first thought that issue must be on Gmail side. But now I can`t even reply to travel agent, who has nothing to do with gmail.com mail address – my mails are blocked.
I have not set up any additional measures for my domain (not even sure what you are talking about 🙂 )
Really need help here .. 🙁