Ubuntu 8.04 LTS with 6.0.4 GA Open Source install of Zimbra. Largely stable and usually very few problems (the zimbra team really are rockstars IMO).

But I'm having kind of a weird issue. I think it might be DNS related, but can't seem to narrow it down. Here's the details:

Topology:

Internet
Router - 192.168.0.0
192.168.200.2 - Barracuda (in/out hybrid - passes all incoming mail to Zimbra)
192.168.1.7 - Zimbra server (passes all outgoing mail back to Barracuda MTA)

This all works normally, but whenever I reboot all mail starts going into the deferred queue with the "Mail Transport Unavailable" error.

after reading through various other similar posts I haven't found an exact match to my problem. Here is the typical information requested from other cases on these forums:

cat /etc/hosts

Code:
127.0.0.1       localhost.localdomain   localhost
127.0.0.1       mail.vpsupply.com       mail
192.168.200.2   barracuda.vpsupply.com  barracuda
cat /etc/resolv.conf

Code:
search corp.vpsupply.com
nameserver 192.168.1.10
nameserver 192.168.1.24
**I should note here that all other DNS names on our network have "corp." in from of our domain. The mail server does not intentionally so that it can interface with our website on the back-end more easily.

dig vpsupply.com mx

Code:
; <<>> DiG 9.4.2-P2.1 <<>> vpsupply.com mx
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 13487
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

;; QUESTION SECTION:
;vpsupply.com.                  IN      MX

;; AUTHORITY SECTION:
vpsupply.com.           3600    IN      SOA     vp_supply.corp.vpsupply.com. hostmaster.corp.vpsupply.com. 14 900 600 86400 3600

;; Query time: 0 msec
;; SERVER: 192.168.1.10#53(192.168.1.10)
;; WHEN: Tue Feb 23 11:42:20 2010
;; MSG SIZE  rcvd: 92
dig vpsupply.com any

Code:
; <<>> DiG 9.4.2-P2.1 <<>> vpsupply.com any
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 62058
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 2

;; QUESTION SECTION:
;vpsupply.com.                  IN      ANY

;; ANSWER SECTION:
vpsupply.com.           3600    IN      NS      vp_supply.corp.vpsupply.com.
vpsupply.com.           3600    IN      NS      capitol.corp.vpsupply.com.
vpsupply.com.           3600    IN      SOA     vp_supply.corp.vpsupply.com. hostmaster.corp.vpsupply.com. 14 900 600 86400 3600

;; ADDITIONAL SECTION:
vp_supply.corp.vpsupply.com. 3600 IN    A       192.168.1.10
capitol.corp.vpsupply.com. 3600 IN      A       192.168.1.24

;; Query time: 0 msec
;; SERVER: 192.168.1.10#53(192.168.1.10)
;; WHEN: Tue Feb 23 11:42:55 2010
;; MSG SIZE  rcvd: 160
host `hostname`

Code:
mail.vpsupply.com has address 192.168.1.7
In order to fix this, I typically have to reboot my Barracuda, then go into the zimbra admin panel under servers>mail.vpsupply.com select the MTA tab, and under "relay MTA for external delivery" switch between "barracuda.corp.vpsupply.com" and "192.168.200.2"

It seems silly, but after doing that toggle, then a "zmcontrol stop/start" the queue flushes.

Like I said, I'm suspecting DNS issues, but then why would it work toggeling from the IP to the DNS name, and not ONLY work with the IP??

I'd like to solve this permanently so that I can work in an auto restart to our monthly maintenance for fsck's.