Error sending emails - errorsendmessage

I see the attached ErrorSendMessage every 2-3 emails I send at the moment. Just started happening the other day.

If I hit re-send enough times, it will finally send. Does anybody else see this or know how to fix it? Driving me up the wall and, at times making mailspring useless to me.

Screenshot 2023-09-24 at 9.50.37 pm