Works now as expected without any additional dependencies!

Discussed with @dvratil that an additional dependency is not the optimal way to solve this issue. Will be halted back until further investigation.

Yes, since it is enabled by default.

Now detached signatures, created with S/MIME can be sucesfully verified.

This workaround produces now the two desired files in S/MIME encryption. The Dialogue also displays correctly that the Encryption and the Creation of a Signature was Successful.

Confirmed. I created a filter to play a beep on every encrypted (tested for non encrypted messages, too) message. When run directly from the filter settings it does the correct thing.

Unfortunately this doesn't work. I set up a filter to play a beep sound on every mail in every subfolder.

Running a filter from context or the settings menu doesn't apply the filter.

Can run and apply filters now from the filters settings

Created a filter to decrypt and encrypt a message and run it directly from the filter settings window.

confirmed in a test.