Got it again this afternoon with a residential customer. Error message on his Windows XP machine with Outlook Express and Norton 360:
"452 4.7.0 Too many messages sent"
The easy fix here was to turn on SSL and use port 465 for outbound, checked "my server requires authentication" and have it use his existing Insightbb email address.
AND - just got a call from the owner of the company who was having problems scanning to email from their copier. He has 16 messages in his Outbox in Outlook. How much you wanna bet it's the same issue?