Sending the previous mail has failed

Hello - periodically (it kind of feels like weekly, but haven’t had enough weeks to confirm that yet) when creating / replying to tickets i get this error/problem.

The RT System itself - Sending the previous mail has failed. Please contact your admin, they can find more details in the logs.

I cannot “see” anything wrong in rt.log, /var/log/maillog (centos7) or apache error_log?

Is there anything that all of the failed communications have in common? Are they all related to a single ticket, or single email address, or single something? Do they fail at the same time of day? Is there anything in any log file happening when the emails fail?

I had these issues with my RT because it was running out of memory. Currently restarting my 2 RT-Prozesses every day because I don’t have time for debugging.

EDIT: I used https://github.com/sysstat/sysstat for monitoring my memory usage.