This has now happened on both my sites.
Upgrade to 3.7.
Using GMAIL relay for SMTP. This has worked perfectly for over a year now.
Testing email settings shows succesful. My SMTP session limit is set to 200 - this fixed a previous issue with Gmail shutting down due to to many sessions being opened.
However, when sending a forum post to a large number of people, after about 120 emails go out, I get a smtp connect failed for all the other emails. This is a huge problem. If I change to phpmail, my emails all get marked as spam. I did change to phpmail on one of the sites however and that did fix the error. On my second site though, I really do not need the spam issue as we will not be able to contact people that we need to so I need to keep using Gmail if possible.
Does anyone have any idea of what I can do? Where I can start? I have searched through various server logs but yet to find any indication there.
One site is on PHP 7.3 and one on 7.1. The only change has been the upgrade from 3.6 to 3.7.