Improvde sender resolution

* src/mail.cpp (Mail::updateOOMData_o): If both sender and
sendusingaccount can be resolved check if we have have a
key for sender and use that.

The logic is that when both sender and sendusingaccount are
found and look like smtp addresses (they don't start with a slash)
then we take the sender address if a key for that address can
be found. Otherwise we use the accounts address as before.

This is important for Institutions that have sender addresses
that differ from their accounts address.

• GnuPG-Bug-Id: T4940