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ThorinOFlag for United States of America

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Sendmail server rejecting our Email (553 5.3.0 user unknown) but the user is good

We host Email for our clients, we also have a Barracuda 800. When our users Email 2 domains which are both hosted by the same company they bounce back with 553 5.3.0 user unknown.

hey are running Sendmail 8.13.6.20060614/8.13.6, that is what I get when I telnet to their mail server.

Our customer thinks that the issue is related to our Barracuda because that's where the bounce message is coming from. Anyone know what the issue is with their Sendmail or if we can do anything on our Barracuda?
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Neil Russell
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Can you post a COMPLETE bounce message here please?
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ASKER

<user1@sendmaildomain.ca>: host sendmaildomain.ca[198.65.45.240] said: 553
    5.3.0 <btv1==950d132e34c==user1@ourdomain.ca>... User unknown (in
    reply to MAIL FROM command
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patterned

We need to see error logs.

Assuming a default setup:

tail -f /var/log/syslog

Then send the message and see what error pops up when it hits.
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Unfortunately I don't have control over the Sendmail server, that is on their end. I was hoping that someone knew of something without having access to it or maybe some known bug.
Well, in my experience, user unknown generally means you're spelling the address wrong.. or it just doesn't plain exist and the employee/user has left, so IT removed the address.

Have you verified the address spelling?
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Ya, I can send to the user from another domain and they can reply back. I know the user exists.
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patterned

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The strange thing is that the error is Following Mail from i.e. when the sender is identified.

You need to look at the sendmail log /var/log/sendmail on what is going on with it.
Understandably you are masquerading the domains of both sides.

Is btv1==950d132e34c==user1@ourdomain.ca a valid email address on the domain?
I.e. can you send emails to this address <btv1==950d132e34c==user1@ourdomain.ca>?
The bounce message is NOT coming from the Barracuda but from the sendmail server.
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They were blacklisting these domains.