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When any account @domainA.com sends to a specific address (eg. b@domainB.com) it bounces. When they send to another address at the same account (eg. c@domainB.com) it goes through just fine. c@domainB.com has existed for a number of years now, b@domainB.com is relatively new, and has had this issue since created.

b@domainB.com can receive emails from any other domain, just not domainA.com.

I set up a forward address (call it d@domainB.com) which passes the email on to b@domainB.com as well as my work account which is on another domain. When a@domainA.com sends to d@domainB.com, I receive the forwarded email, but b@domainB.com does not.

My questions.. More generally, what in the world is happening here? Specifically, is this issue something on the receiving end or the sending end? and how do I fix it if it's on the receiving end (which I control) or what do I tell the guys on the sending end to look for?

Here is the message received by domainA.com users when they send to the account:

Your message did not reach some or all of the intended recipients. Subject: Test Sent: 1/29/2010 8:25 AM The following recipient(s) cannot be reached: 'b@domainB.com' on 1/29/2010 8:25 AM 550 unknown user

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What mail server? –  Zoredache Feb 2 '10 at 18:05
    
Your process is well thought out but we really need to know what email server. Also, I bet anything you left out part of that error message. Sounds like b@domainB.com is attempting to deliver locally. What shows up in the logs of the second email server? Make sure it's really bouncing the email back after first server attempts delivery. –  Patrick R Feb 2 '10 at 18:17
    
That's the whole error message as forwarded to me by the user @domainA.com I don't know what mail server a@domainA.com is using, but I will try to find out. The recipient is running on a Linux box with cPanel/WHM for admin functions. As for the logs on the recipient server.. nothing. I've staged tests with the different users and searched through the logs to find it and nothing shows up. –  Jonathan Hack Feb 2 '10 at 19:00

2 Answers 2

Given the error 'unknown user', it looks like the 'b' account is setup improperly. Or something about that account is configured incorrectly.

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Yes, but the user is only unknown to one sending domain. Everyone else can send to it. So it seems to me like it is set up properly, but I could be wrong. Any ideas on how I could troubleshoot it to determine if something is misconfigured? It was created using cPanel just like all the other accounts (which work fine). –  Jonathan Hack Feb 2 '10 at 23:06

Check whether your SMTP account requires authentication, not SSL.

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