Test email works but email notifications do not
I cannot figure out why I do not receive any email notifications about a changed issue status even as a Watcher / Assignee. I've tried both as a member in the project and non-member. The project is set to public and I have set the settings in /settings?tab=notifications to 'Issue added' and 'Issue updated' and in /my/account to 'For any event in all my projects'. Test email sent from admin tab goes through and works as intended but I cannot for the life of me figure out why the email notifications do not go through. Any ideas where to start debugging the issue? I tried browsing through the production.log but didn't find anything that would point to errors in sending emails.
Redmine version 3.4.6.stable
Ruby version 2.3.7-p456 (2018-03-28) [x86_64-linux]
Rails version 4.2.8
Database adapter Mysql2
Thanks for any help in advance!