I was talking about inbound mail to the Zimbra server. If you mean outbound mail, that's nothing to do with Zimbra as it depends on the receiving server settings.
Okay, a user here tried to send an email to a fusemail account but it was deferred I think Fusemail was having some issues as they normally are. I hit re-queue and they sent fine. How often will Zimbra try to resend those emails.