Add port_check_remote() & use it in smtp fixture

py.test -k test_email with firewall up & down to see the difference.

Currently, it fails with "TimedOutError: Could not do function <lambda>() in time" which is quite ambiguous.

This if clause needs to be before the ``if port`` line 83, else addr is None during the check but the result gets stored in the proper place, meaning that the connection is tested every time.