Hi, We have intermittent 550 5.7.1 Unable to relay errors when email is being sent to certain email addresses with a certain domain name in our exchange server (Exchange server 2003). Email addresses with other domain names work fine. i.e. email to anything@a.com always gets successfully delivered. email to anything@b.com gets intermittent 550 5.7.1 errors. The server that is trying to send the messages is in the exchange servers permitted relay list in: [Server]->Protocols->SMTP->Default SMTP Virtual Server->Properties->Access Tab->Relay We also have "Allow all computers that successfully authenticate to relay, regardless of the list" enabled. Can anyone point me in the direction of where to look as we are scratching our heads on this one. Thanks in advance.

Versions involved? I can take a guess, but you haven't stated what they are for sure. The screens could be two or three different ones. Do you have SMTP logging enabled, so you can verify that they are connecting on the expected IP address? Are you able to use authenticated relaying? That is my preferred control mechanism as it can be locked down. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources | In the UK? Hire Me.

Hi, I was just editing my original message when you posted. It's exchange server 2003. Yes we do have SMTP logging enabled and they are connecting from the correct ip's. Our email coming into this machine is coming from an external perimeter system so it always arrives in from one of 2 possible ip addresses which are both in the permitted computers relay list. These machines always successfully deliver email to the other email address domain anything@a.com so we don't think it can be an issue with the machines sending the email in to us not being allowed connect and use exchange to relay. We think that it is specific to the way that our exchange system or AD is handling the routing for the b.com email addresses. Any ideas?

Hello, It seems the issue is with the anything@b.com domain. Try registering another email address, such as GMAIL, and send a test email to anything@b.com. Thanks, Simon