Please read this post: http://www.zimbra.com/forums/adminis...-ironport.html
and this attachment: http://www.zimbra.com/forums/attachm...rt-mta-ukw.pdf
"Simple external live e-mail backup with exim solution:
Users can add a external imap server in Zimbra client and get the all backup incomming and outgoing messages with exim server infrastructure. You can also get the backup messages with standard client, like MS Outloock or Thunderbird (use the imap cconnectiom to exim server)."
Exim (smtpd) can backup all inside and outside e-mail on external server. You just have all mail traffic routed through it.