450 Server configuration problem

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'abcdefg@hotmail.com'. Subject 'Re: Resending of Logo Art.', Account: 'my acct (mydomain.com)', Server: 'mail.mydomain.com', Protocol: SMTP, Server Response: '450 Server configuration problem', Port: 25, Secure(SSL): No, Server Error: 450, Error Number: 0x800CCC79

May I get help on how to rectify this issue? We could not send out any emails since 10am (GMT+8) in the morning. I read through the support pages on email config and could not find the solution. Please offer some advice. Thanks

I'm seeing the same problem. Manifested itself in Mail.app reporting an ambiguous "problem" sending (Otherwise, it's a decent client, but I wish it'd give out "real" error messages).

Telnetting to port 25 and attempting to send a test message to myself manually resulted in the 450 error.

Apparently at this time, DH has been aware of the problem for ~4 hours, at least when I checked via support.

Sending via webmail seems to work, so there's a workaround. Annoying, but not critical.