You receive non-delivery reports (NDR) when you send e-mail attachments that are larger than a specific size in Exchange Server

This one is interesting to my alter-ego…

Consider the following scenario. In a Microsoft Exchange Server organization, the Exchange Server server has no size restrictions for e-mail attachments and no quota settings on mailboxes. Additionally, size restrictions are set on the firewall.

In this scenario, you may receive non-delivery reports (NDR) when you send e-mail attachments that are larger than a specific size. For example, you may receive an error message that resembles the following error message:

—– The following addresses had permanent fatal errors —–
<user@domain.com>

—– Transcript of session follows —–
… while talking to mail.domain.com.:
>>> DATA
<<< 552 Requested mail action aborted: exceeded storage allocation
554 <user@domain.com>… Service unavailable

Final-Recipient: RFC822; user@domain.com
Action: failed
Status: 5.2.2
Remote-MTA: DNS; mail.domain.com
Diagnostic-Code: SMTP; 552 Requested mail action aborted: exceeded storage allocation
Last-Attempt-Date: Fri, 2 May 2003 16:59:59 -0400 (EDT)

The above error is actually being caused by the *Firewall*, not by any storage allocation limits.

Source: http://support.microsoft.com/default.aspx/kb/944281

It is unfortunate that the error generated does not give a true indication of the cause; if you were a technician diagnosing this error, you’d be led astray by the message.  A google search for SMTP 552 is similarly unhelpful.