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?