I've been having an issue with messages bouncing back when I send them. It puts a copy of the message into the Drafts folder. When I open the message to resend it, it usually goes right through. Sometimes it bounces back a couple of times, but invariably goes through.

What do I need to look at to troubleshoot this?

Bounce notification subject:
Delivery Failure Notification: Invalid address: Joe Blow . com.zimbra.cs.mailbox.MailSender$SafeSendFailedExc eption: MESSAGE_NOT_DELIVERED; chained exception is: com.zimbra.cs.mailclient.smtp.InvalidRecipientExce ption: RCPT failed: Invalid recipient joe@blow.com: 554 5.7.1 : Relay access denied
Bounce notification body:
Your message "Re: Cool subject line" sent on March 13, 2012 at 12:23 PM to "Joe Blow " can't be delivered. None of the recipients will receive the message. It has been returned to Drafts folder for your review.
I mostly don't understand why it's telling me "Relay access denied" but then seconds later will allow me to send the message.

Thanks for any advice!