Add fallback for sender email

In case the sender email addess is not an smtp address fall
back to the property accessor.

* src/ribbon-callbacks.cpp (do_reader_action): Use property
accessor for non smtp addresses.

• Here the property accessor returned an unknown property error when the sender address was smtp. But according to documentation the Sender object should have that property for internal exchange senders. This also just omits the sender from the verify call (as get_pa_string returns NULL on error) if the sender address is not smtp and can not be obtained with the property.