Using v 8.17, the project-level notifications suddenly stopped being delivered to the recipients. Project notifications on ticket update, comment, even when mentioning another user, stopped working however. Admin mail (via send email) and filter subscriptions suddenly stopped working without any changes from admin side I am aware of.
Checking the logs, I identified a couple of invalid users with non-existent email addresses. These users got removed and the tickets in question reassigned.
I also changed back to the default notification scheme for testing purposes and checked the notification helper, which indicates test persons should indeed receive notifications.
I am aware of the following help page but could not find a solution that works for my situation: https://confluence.atlassian.com/jirakb/troubleshoot-notifications-in-jira-203394737.html
