Still no notifications, tested again with a freemail account (web.de) and my own business domain.
Notifications from KRP forum still work without problems and it uses the same forum software.
The sending mailserver was smtp1.servage.net
Didn't you use this mailserver also for PiBoSo forum when it still worked? Did you change the SMTP server back to web05.i3d.net which caused problems in the past?
The forum became unreachable 10 days ago.
After re-uploading all the forum software, it restarted working, but the emails weren't working anymore.
Then tried to switch to web05.i3d.net, and tried to make it work.
One last try that can be done is indeed to go back to the servage SMTP server.
Please report if it works now.