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I have a Microsoft Exchange Server 2003 installed in my office. It was configured with all MX-records and SMTP relaying recomendations required to gain trust and not have my server blacklisted.

It is also aparently not blacklisted in any email server blacklist I can find online, and in overall it works. I can send emails to Gmail, Hotmail, and most other email servers.

Howover, some few email servers insist in not receiving my e-mails for some random reason I couldn't so far understand. There is no error, they simply drop it.

Is there something I can do to get this mystery solved?

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What does your SMTP server log say for those emails that get dropped? –  EEAA Sep 7 '12 at 2:36
The email is sent. –  Havenard Sep 7 '12 at 2:38
Does the receiving server return a 250 code? If so, then they're not dropping the message. 250 means "message accepted". Now, your messages may be getting stuck in their spam filter. If that's the case, then you just need to make your emails look less spammy. –  EEAA Sep 7 '12 at 2:41
I'll make some tests tomorrow, but I'm pretty sure they silently drop the emails the same way they would if I was blacklisted. Its not a matter of how the message itself looks, a plan text message suffer the same behavior. I know because when I was installing the server I had a lot of headache with blacklists that were listing my IP even before I owned it. –  Havenard Sep 7 '12 at 2:47
If you were blacklisted, you would not be silently dropped, and you most certainly would not be getting a 250 code back from them unless they had a very evil email administrator. –  EEAA Sep 7 '12 at 2:48

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