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Our environment:

  • Exchange Server 2003 Standard, single instance, running on Windows Server 2003 Standard.
    • configured to not send/receive mail with attachments larger than 10 MB.
    • NDRs are not enabled.

The issue:

When an external sender sends an email with an attachment larger than 10MB, Exchange, as configured, does not receive the message. However, the sender of that message does not receive any notifications from his own mail server that the message could not be delivered due to attachment size.

However, if an external user tries to send an email to a non-existent user, they do receive a message from their mail server indicating that the user does not exist.

Why is that, and is there anything I can do about it? It would be nice if the sender received notification that the attachment file size exceeds our limits and their message was never received...

Update

The Exchange server has a SpamAssassin box in front of it... could that have something to do with it?

Here is one of the last lines from SpamAssassin's logs when searching for my test e-mails:

mail postfix/smtp[19133]: 2B80917758: to=, relay=10.0.0.8[10.0.0.8]:25, delay=4.3, delays=2.6/0/0/1.7, dsn=2.6.0, status=sent (250 2.6.0 Queued mail for delivery)

My assumption is that Spam Assassin thinks the message is OK and is forwarding it off to Exchange.

Update

I've verified that Exchange is receiving the message and generating an NDR. However, delivery of NDRs are disabled to prevent Backscatter. Is there something that I can do to get Exchange to send a bounce message to the sending mail server (or verify that message is being sent) so the sending mail server can notify its sender of the bounce?

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Well it's probably fair to say that until you're sure which one of the boxes is dropping the attachments then it will be difficult to troubleshoot it. If you send a large test email to yourself then review the logs on the spam assassin setup then does it show the email as being forwarded to exchange? If it's exchange that's dropping the email then could spam assassin be filtering the NDRs? –  RobM Oct 6 '10 at 21:44
    
@Robert: Fair statement. I have to admit, I know little about SpamAssassin, however, I've updated the question with more about SpamAssassin's logs. –  Cypher Oct 7 '10 at 1:20
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1 Answer

If we're sure that Exchange is doing it then you can set more comprehensive logging on the SMTP connector and so-on to hopefully see what is going on.

It's been a long long time since I last used Exchange 2003 so I can't remember the details, but it isn't difficult to find when you look at the properties of the connector. You can also enable message trackinglink text. You should be able to establish if a new test message is accepted or silently dropped with these tools, or if exchange is sending a NDR and it just isn't arriving - which probably takes you back to the spamassassin box.

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I've verified that Exchange is receiving the message, and generating an NDR. However, NDRs are not sent outside the domain. Backscatter isn't cool. :-) NDRs shouldn't be necessary here, as it isn't necessary for users who send mail to our domain to an address that doesn't exist still gets a reply from their sending mail server that the user doesn't exist with the SMTP error and all - I'm more curious about why that isn't happening with the large attachments. Is that not part of the SMTP protocol? Shouldn't Exchange be generating a bounce message to send to the sending server? –  Cypher Oct 7 '10 at 16:45
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