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I believe the purpose of the question is not just Barracuda but to understand the system better.

Lets say your company email is blocked by Barracuda security system when you sent an email to a friend at his company. Your email cannot be delivered because they recognized your email address as spam (lets say your company sends a lot of emails as most companies do). Now we know that it could also be because of a virus on the computer and that is why your email is blocked. Ok that makes sense.

But if I sent the same company an email from the same computer with yahoo or gmail, it is delivered successfully?

My question is what stamps are used by Barracuda to block you? How does the system work. Doesn't Barracuda identify spammers by their IP address? If that is true, why one IP address can send email from one account but not the other?

Does anyone has insight into it and what is the best way to resolve this issues?

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Anyone who downvoted this, can explain by his/her genius? what is wrong with this question and why it is not a fit here? –  Dave Nov 23 '11 at 15:20
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1 Answer

How it works:

IP based spam filters (like Barracuda's) do not care about the IP of the message's sender but the mail server.

If you send an email using your company's email address, Barracuda checks the IP of your comapny's mail server.

If you send an email using GMail, Barracuda checks the IP of GMail's corresponding mail server (which obviously is not blacklisted).

How to avoid blacklisting:

Even if your company never sent spam, it is possible that it got blacklisted.

The simplest way to wind up on Barracuda's blacklist is an inconsistent (or non-existent) Reverse DNS entry.

If smtp.server.com maps to 1.2.3.4 (DNS entry) , 1.2.3.4 should map to smtp.server.com (Reverse DNS entry).

If your company is falsely listed, your system administrator can request delisting on the Barracuda website.

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I did not quite get it. This is how you can be blacklisted on Barracuda? Or it it some kind of resolution to the problem? –  Dave Nov 23 '11 at 15:18
    
I re-read your question and realized that I did not answer an important part. I will update my answer. –  Dennis Nov 23 '11 at 15:23
    
Just for my information. so it is mail server that the block is based on, not on your computer IP level. Of course it could be based on your IP as well (if you send 1000 email from different servers) or computer IP is never blocked? you have any insight? –  Dave Nov 23 '11 at 15:28
    
No, it is not based on the sender's IP. That would not make any sense, since most end-user IPs are dynamic. If you send spam thorugh your GMail account, it is GMail that has to block you, not the recipient. –  Dennis Nov 23 '11 at 15:30
    
That makes sense. So I would say it is the your mail server name and mail server IP that lands you on barracuda list. Because we use shared hosting and that means many other companies will have the same IP for mail server as we do? Or I am wrong? –  Dave Nov 23 '11 at 15:44
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