Better misconfigured RELAY_TX handling

If you should happen to leave RELAY_TX configured in sbbs.ini mail options but don't configure an actual relay server, when email is sent an error about not being able to resolve the hostname is logged.

Dec 23 20:51:08 bbs synchronet: mail 0000 SEND Message #10211 (708 of 709) from 'Nelgin' #1 (closed) to 'hidden' hidden@hidden.net Dec 23 20:51:08 bbs synchronet: mail 0000 SEND getting message text Dec 23 20:51:08 bbs synchronet: mail 0082 SEND resolving SMTP hostname: Dec 23 20:51:08 bbs synchronet: mail 0082 SEND !Failure resolving hostname: Dec 23 20:51:08 bbs synchronet: mail 0082 SEND !Delivery attempt #6 (closed) FAILED (Error resolving hostname ) for message #10211 from Nelgin to hidden@hidden.net

While this is somewhat accurate, if RelayServer is not defined there is no server to lookup in the first place and would prefer to see this caught and logged if RELAY_TX is set.

Maybe something like "Relay host is not defined".