Fixed. Turns out that SMTP Tweak in WHM, which was working correctly before new update, blocked connections to Exim after the cPanel update. Our support guys turned it off and now everything works.
Turning off the SMTP tweak worked for me also
Just checked my mail queue and there was a build up of regular mail all with a "Connection refused" error.
This looks like it started after a manual update to WHM 9.9.8 cPanel 9.9.8-E81 on Sunday am.
Turning off the SMTP Tweak
in WHM/Server Setup/Tweak Security
also fixed this issue.
All mail was sent from the queue and email back to normal.
This was all working previous to this last update.
Once again Forum Search
to the rescue which facilitated a fast problem turn around.