I ran into the following issue this week while traveling on business to Las Vegas: Sending e-mail from Outlook 2007 was failing with the error:

None of the authentication methods supported by this client are supported by your server.

I thought this was odd, as I hadn’t changed anything with the configuration — just a different network. I hunted around, and determined a possible fix was to disable the “My outgoing server (SMTP) requires authentication” option on the Outgoing Server tab in Outlook. This worked! Strange.

Coming back to Seattle, I was unable to send e-mail again, with the following (different) error:

550 5.7.1 Unable to relay for <email>

Turning the “My outgoing server (SMTP) requires authentication” back on fixed this problem.

This seems like a very strange problem. It has to be network related, but what about the particular network would cause this? Dunno.