We've had a few verified instances (verified as much as I can, really) where an issue was assigned to someone and they didn't get an email notification. After the first couple reports, and all the logs appeared clean, and the users are swearing Jira's emails aren't in their spam folder or anything like that, I turned on the "Outgoing mail log" (but not at the debug level).
Another report came in this morning; I checked the latest atlassian-jira-outgoing-mail.log and the last entry was written about 20 hours ago, the day prior.
Any ideas on how I can troubleshoot this further? Notifications work 99% of the time, it's this 1% that is being noticed however.
-Kelly