Question : IIS SMTP - "Relaying denied. Proper authentication required."

Hi,
I have a user that is unable to send mail to customers at a  specific domain. His messages are rejected with "Relaying denied. Proper authentication required.".

The site is using Exchange5.5 which forwards to a local PC running GFI Mail Essentials and IIS SMTP. I have examined the logs on this machine and can see this response to RCPT TO command for an  outbound connection to an external IP address.

If a different user on the network sends a email to the same email addresses the the message is successfully delivered. On examining the logs the outbound connection is made to a different IP address from the first users attempt.

Any suggestions most gratefully received,

Andy

Answer : IIS SMTP - "Relaying denied. Proper authentication required."

There is nothing in SMTP to control which server the email is delivered to based on the full email address. Email is only routed based on the bit after the @ sign.
If the domain has multiple MX records then you cannot accurately predict which server will be used.

Simon.