In some configurations, the Messaging Gateway (SMG) mail server process (MTA) may exit with a signal 6 and restart. If alerts are configured this will generate one or more admin alert messages.
Subject: mta crashed on signal 6 on [mail host name]
mta crashed on signal 6 on [mail host name]
exit code: 0x0086
Disable DKIM validation via "Spam > Sender Authentication > DKIM > Enable DomainKeys Identified Mail (DKIM) validation for all incoming mail" and monitor the affected system. This may stop or reduce the frequency with which the process restarts.