getting dozen of mails from nobody@invalid.invalid every day (GMAIL)

happens every morning around 08.00

I red this is some kind of bug, but updating em client beta did not get rid of this issue 

This was fixed in a recent beta version.

If you would like to try version 8 beta, you can download it from the Release History.

I did so:

8.0.1540 (50a7780) is the Version i got

but the problem still exists 

Maybe it does not affect messages you already received. What you can try though is to right-click on the All Mail folder, and choose Properties > Repair.