If application is connected to a Domino mail server via IMAP, it can happen from time to time that a message is expunged.
This leads to a `MessageRemovedException` when calling `IMAPMessage#getSubject`.
And although debug is set to false this again leads to a `MessageException` and the whole integration flow stops.
* Check for `isExpunged()` before logging a filtered message
* Add `@author`
* Fix debug message for expunged to reflect reality
* Clean up new unit tests a bit
**Cherry-pick to `5.4.x`**
c189a125a6