Hey guys,

Very strange behavior here. Everything was running smoothly for some time now, until we received a complain from the users and it turned out the the server is deferring emails...
It is a 8.0.3 version of Zimbra running on Ubuntu.
Here is some info from the logs:

May 28 17:09:02 mail postfix/error[10310]: D74F618CBA6E: to=<user@domain.xxx>, relay=none, delay=0.08, delays=0.07/0.01/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)

zimbra@mail:/home/user$ dig domain.xx mx

; <<>> DiG 9.7.0-P1 <<>> domain.xx mx
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 31511
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;domain.xx. IN MX

domain.xx. 57849 IN MX 10 mail.domain.xx.

mail.domain.xx. 57954 IN A xxx.xxx.xxx.xxx

;; Query time: 3 msec
;; WHEN: Tue May 28 17:11:00 2013
;; MSG SIZE rcvd: 65


It's a multi-domain installation and all emails to external recepients are deferred. There is only one exclusion - admin@mail.domain.xx (the default admin account) can send mail to external recepients. All users from the other domains can send to domains internal to Zimbra, but not to any external domain.

Any idea?