have a Zimbra Ver2 Mail server setup behind a firewall.
Mail keeps getting stuck in the postfix mail queue

The server is on a private IP address the firewall is on a public ip address.

The following ports are forwarded to the private ip,

Zimbra_Admin 7071

Zimbra_HTTP 80

Zimbra_HTTPS 443

Zimbra_SMTP 25

Zimbra_SSH 22

Zimbra_IMAP 143

Zimbra_IMAPS 993

Zimbra_POPS 995

Zimbra_POP 110

Zimbra_LDAP 389

Zimbra_Contr 7777

Zimbra_?? 7025

The /etc/hosts file has for the ip address of the domain name

when the server is first started, everything works as expected.

after a couple hours running, the local mail stops getting delivered, with this error message in the zimbra.log

status=deferred (connect to domain.com[123.123publicip]: read timeout)

'zmcontrol status' shows everything as running.

server shows 200~300 mb free memory

when I try to do a postqueue -f (Flush queue) get the same timeout errors.

Only way to fix it is to do a zmcontrol stop, zmcontrol start, then postqueue -f this then delivers the mails

I don't understand why this is happening.