Redmine

We have the issue that non of our Users gets a E-Mail anymore if something in their Tickets changed.
We did not change anything nor installed a new version.. It just happened from one day to another that no one is getting E-Mails anymore.
The standard E-Mail Option in the users menue ist still activ for everyone.

This is our environment:

Environment:
Redmine version 4.2.1.stable
Ruby version 2.7.3-p183 (2021-04-05) [x86_64-linux]
Rails version 5.2.5
Environment production
Database adapter Mysql2
Mailer queue ActiveJob::QueueAdapters::AsyncAdapter
Mailer delivery smtp
SCM:
Subversion 1.10.4
Mercurial 4.8.2
Bazaar 2.8.0
Git 2.20.1
Filesystem
Redmine plugins:
redmine_agile 1.6.1
redmine_checklists 3.1.19
redmine_ckeditor 1.2.3
redmine_custom_workflows 1.0.5