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: D74F618CBA6E: to=<firstname.lastname@example.org>, 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
;; QUESTION SECTION:
;domain.xx. IN MX
;; ANSWER SECTION:
domain.xx. 57849 IN MX 10 mail.domain.xx.
;; ADDITIONAL SECTION:
mail.domain.xx. 57954 IN A xxx.xxx.xxx.xxx
;; Query time: 3 msec
;; SERVER: 10.1.0.10#53(10.1.0.10)
;; 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 - email@example.com (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.