Hi there.

Our mailboxes for asmucol.org are configured using domains.live.com.
We have used this service for more than 2 years, with no problems. 
However, since yesterday we have been getting this message every time we send an email to a user in the same domain:

This is an automatically generated Delivery Status Notification.
Delivery to the following recipients failed.
       *** Email address is removed for privacy ***

Looking a the message source code, i found:

Final-Recipient: rfc822;*** Email address is removed for privacy ***
Action: failed
Status: 5.5.0
Diagnostic-Code: smtp;550-Please turn on SMTP Authentication in your mail client, or login to the
550-IMAP/POP3 server before sending your message.
550-blu0-omc1-s19.blu0.hotmail.com [65.55.116.30]:10861 is not permitted to
550 relay through this server without authentication.

Which is really wierd, as we are not using a mail client. 

It seems to be a hotmail issue with the outgoing emails to @asmucol.org, as i am able to send emails to other domains like @hotmail.com, @gmail.com or @yahoo.com.

Hope you can find and fix the problem.

Thanks a lot.