I just had a user send an email to a client 9,000 times. The client isn't very happy. This happened a month ago with a different user as well.
What the user said she did was reply to an email, enter a few lines of text, then click send. After a half hour, she noticed the email was still in the outbox. She exited outlook, then went back in, it still wouldn't go out. She rebooted, then was able to remove the email from the Outbox. Then she had about 500 undeliverable email bounce messages from the client's mail server. (They have some sort of throttling thing.)
I've turned on zimbra connector logging, so if this happens again, I'll get a log. I've informed all users to make sure their outboxes are empty after sending an email.
This is with 6.04 client and server. I've upgraded her client to 6.06. I'll be upgrading the server this weekend to 6.06.