This error occurs in both Outlook Express and Entourage when the message being sent is too large to be accommodated by the outgoing mail server. The upper bounds for acceptable message sizes are set on the mail server itself, not by the individual user. If possible try to break apart or compress the attachment(s) before sending the mail again. If you need to send large attachments on a frequent basis, you may need to consider using a different e-mail provider that supports a larger maximum message size.