Error - 17099

You are using Entourage I take it? Which version? The error seems to be coming from your ISP. Have you sent a lot of emails from that account in the last 24 hours? How many recipients are there for his particular message?

Here is description if the error:

Error -17099
Relaying to <the email address> prohibited by administrator
Email could not be sent
Account name (my account name)
Error -17099

Error -17099 is an Entourage error kSMTPServerReplyErr, and is returned when the SMTP server has returned a permanent or transient error or returns an unexpected result code.

This error is coming from your ISP.

Users have reported this error when sending to large groups. ISPs limit the number in a group. Usual number is 50 recipients to avoid spammer misuse. Entourage has no limit. You should contact your mail provider to ascertain the maximum number of recipients per message and note it. You can use the free "Split Recipients" script to send a mass e-mail in separate mailings automatically

G4 1.42GHz Dual Processor (home) Mac OS X (10.4.8) eMac 1.42GHz (work) 60BG Black Video iPod

How many people was this message addressed to? Were there any groups it was being sent to? Who is your ISP?

Have you looked under Personal Folders -> Outbox to see if anything is there that never sent? That would explain it trying to send messages every time you go online and if there are large messages that are trying to send every Send & Receive schedule that may very well be causing your issue.