I thought this would be fairly common knowledge by now, Exchange 2003 being quite mature in it’s 5th year, but it’s not something I’ve had a problem with before and therefore I’m going to write about it!
So a big email comes in; lets say it’s 8MB. Your Exchange 2003 server, set to it’s defaults for size restrictions, rejects the email. Why? Take a look at this Exchange TechNet article:
When the 8MB message crossed the routing group boundary through SMTP and arrived at the destination server, it was approximately 33 percent larger than the original message because of the inter-routing group SMTP increase…The final message had a content size equal to 11,594,558 (11 MB), and the message exceeded the 10-MB Global Limit, thus returning the 5.2.3 delivery status notification.
Please keep in mind that message send [sic] through SMTP could grow about 10-20 percent because of format conversion (MIME and UUEncode)
For a standard Exchange Server installation, this is how the process of checking the email size goes (see the diagram below for full details):
You can set message limits at the following objects: