I'm stuck with the following error, after I've triple-checked with the webhost that I've got the external smtp mail settings right. Apparently, C5 requires authentication, where the webserver does not. Just putting the password does not work, and a missing password apparently does not turn it off either.

Here's the log description:
Mail Exception Occurred. Unable to send mail: 503 5.5.1 Error: authentication not enabled

Any clues?