We have an MTA that needs to relay inbound mail to an Exchange server. I found that when the MAIL FROM address matches the username in the AUTH credentials, Exchange accepts the email. However, when they don't match, Exchange rejects the email with an error like this:
550 5.7.1 Client does not have permissions to send as this sender
Our MTA only allows us to specify one set of credentials; all emails, regardless of the sender, must be relayed to the Exchange server using the same set of authentication (AUTH) credentials.
Is this possible? I hope it is, because this requirement is coming from my Director.