I get this error fairly frequently:
Delivery Failure Notification: Invalid address: Joe Blow <joe@blow.com>. 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 <joe@blow.com>: Relay access denied
What is concerning about this is I can resend it and it goes through right away.

A sample exchange usually goes like this:

8:00 I send email to 'Joe Blow'
8:00 I get a Delivery Failure Notification
8:01 I resend email (from drafts) to 'Joe' without changing anything, mail goes through
8:05 Joe replies
8:10 I reply, it goes through with no issue
8:12 Joe replies
8:13 I reply and get a 'DFN'
8:14 I resend and get a 'DFN'
8:15 I resend and it goes through with no problem

Of course it doesn't happen exactly this way every time, but usually once or twice a day I get failures like this, but some days there are no failures.

This happens whether I am using the web client, the Zimbra Desktop, or my iPhone.

What could be causing all these bounces when literally minutes or even seconds later it sends just fine?

Any ideas on what to look for would be appreciated!