Hi, I'm running Zimbra 8.0.3 OSE on Ubuntu 12.04 LTS. After switching to a IPv4/IPv6 dual stack setup, it seems that all outgoing mail gets stuck in the deferred queue, seemingly due to a problem with the milter:

Apr 27 11:08:52 mail postfix/smtp[1046]: 1DA9540129: to=<recipient@example.com>, relay=::1[::1]:10026, conn_use=2, delay=390107, delays=390104/2.6/0.04/0.36, dsn=4.7.1, status=deferred (host ::1[::1] said: 451 4.7.1 id=31030-01-2 - Temporary MTA failure on relaying, from MTA(smtp:[::1]:10030): 451 4.7.1 Service unavailable - try again later (in reply to end of DATA command))
If I requeue the mails via the queue management interface, they get sent.