No notifications after 6.12.65 to 10.13.66 track migration

I migrated my 6.12.65 test/internal system to 10.13.66 at the end of December, have applied all module updates since. I’ve just noticed that I’m not receiving any notification emails since that time. That includes voicemail notifications, voicemail pager notifications, backup notification, module update notification, etc. I’ve tested postfix and can successfully send a test email by telnetting to port 25 on localhost. There is nothing at all in the /etc/maillog since the start of the year (aside from postfix stopping and starting).

Suggestions on where to start troubleshooting this further?