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I am trying to get e-mail to a customer of ours. From my internal Exchange server, I can send the e-mial successfully to the customer if there is no attachment. I can send the email successfully to the customer with an attachment (zip, docx, pdf tested) from my Gmail account. If I try to send an e-mail with any attachment (zip, docx, pdf) from my domain, the mail never reaches the customers. All I get from them is a bounce "proxy1.somedomain.com <proxy1.somedomain.com #4.4.7> #SMTP#" (customer domain removed for privacy).

Does anyone have any idea where I can start testing this? It seems to be a problem with attachments from my domain, but why would a server only block e-mail with attachments (any attachment) from my domain only?

Thanks

NOTE: I thought I would add, we have an internal Exchange 2007 server and a Sendmail relay. The sendmail server just takes incoming mail and searches for spam and malware before it relays the mail into my Exhcnage server. The sendmail server doesn't see outgoing mail.

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Did you run some tests against your MX servers like those provided here: mxtoolbox.com ? –  Maxwell May 21 '12 at 16:15
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could be based on reputation. or the way that your server is formatting attachments. Exchange will a lot of times send a TNEF(Winmail.dat) file for attachments if you are set to use rich text, whereas gmail won't. Try changing your mail format to plain text or HTML and see if the problem persists. But Alas I think the real answer is to check with the other mail server admin and find out why they are denying it, since anything else is just guess work. –  Doon May 21 '12 at 16:37
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2 Answers

up vote 1 down vote accepted

I'll go with my default response for this one. =)

The unfortunate nature of current anti-spam techniques is that they are imperfect and prone to false positives if the recipient has settings wound too tight. I don't have a specific answer for you but I think there would be value in trying a few troubleshooting steps:

  • Is this happening for all messages with attachments to that domain from the sender's default email account? Or just some attachments?
  • If the sender re-sends the message later, does it go through? (Just trying to determine if block is due to flow-control or is message specific.)
  • Did this behavior start recently? Or has this been an on-going issue?
  • Have your users ever been able to successfully send attachments to this domain?

If nothing obvious stands out, you'll likely need to contact the recipient mail administrator who can hopefully elaborate on what's going on.

Hope this helps.

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Your last recommendation (contact the servers admin) is actually what I did. They acknowledged that the problem was on their end but unfortunately never got back to me nor did they fix the problem. –  Albion Jun 1 '12 at 14:33
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I had a similar issue recently with a newly configured Exchange Server. One of the last places I looked, seemed to fix the issue... DNS forwarders.

With the case that I had, simply adding 8.8.8.8 as primary DNS forwarder worked a treat.

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