Exchange 5.5 Delivery Errors

Recently I've been receiving complaints about my exchange server 5.5 is bombing on some messages. The error reads....
Your e-mail is being returned to you because there was a problem with its delivery. The address which as undeliverable is listedin the section labeled: "-----Transcript of Session Follows ----" 451 .. replay: blah blah Diagnostic-code: SMTP;451

What does this mean? I checked my exchange server to make sure there were not limits set, and as far as I can tell there is not. What am I missing?

Thanks for any help/guidance.

Michelle
New Mexico School for the Deaf