Exchange 2003 user in a small investment firm sends out a mass mailing to all investors (about 80 recipients) and gets back 4 failure notices. Focusing on one of the failures that says "550 <our mail server> is currently not permitted to relay". This particular investor is upset and wants to know why we can't send him anything when everyone else in the world has no trouble. Emails to him fail every time when they are composed new whether he is the only recipient or a CC. Oddly, replying to his emails works just fine.
I have run our mail server through all the tests on MXToolBox and the only warning I get is "5.476 seconds - Warning on Transaction time" after the smtp diag test. The blacklists are clean, reverse DNS works and our Reverse DNS matches the SMTP banner.
Also, when I run the Microsoft utility, SmtpDiag, using our user's address as the sender and the investor's email as the recipient, it passes every test and sends successfully.
Our Exchange 2003 server is on the same machine, so the are no external DNS servers set up in exchange. DNS forwarders are set to our ISP's DNS servers in DNS.
Not sure if it's significant, but we have Kaspersky Security 5.5 for Microsoft Exchange. We use Postini for incoming, but not for outgoing.
Questions: Is there anything else in the setup on our end that could cause this failure? Is there a better way than SmtpDiag to test sending without actually sending an email to this investor? He's not in a good mood, so I don't want to involve him.