add dkim client capability

[gpatel-fr]

Hello

since I have found a bug in my implementation I am sending another attempt.

the bug is that when returning a delivery error, the sender is not defined, as no test was done in this case there was an inappropriate error message in the logs, hence this added code:

     dom = envelope.sender
     if not dom:
         return  # no warn if sending deliv. fail notif.

there are also a few cosmetic changes (pep8)