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: