FYI: Running Dataverse 4.11
I experienced a hard-to-diagnose issue where an action (e.g. publishing a dataset) would fail, but with no visible indication as to why.
I was able to figure out that our local SMTP relay wasn't online. Once that service was restored, the operations completed normally.
A number of tasks send out email notifications. Even if the SMTP server was offline, I would expect these operations to complete successfully -- but show a warning about not being able to send out the notification.