hello,
we have ZCS 5.02 in ubuntu 6.06, when I check in the:
web console>Mail Queues>mail.domain.com>Deferred
when we try send a email to user@cantv.net, we find the next error

host relay.cantv.net[200.44.32.36] said: 421 4.4.1 collect: read timeout on connection from mail.domain.com from=<user@domail.com> (in reply to end of data command)

in /var/log/zimbra.log:

Jun 11 16:05:53 mail postfix/smtp[7114]: E538898813F: to=<receive.user@domail.com>, relay=relay.cantv.net[200.44.32.36]:25, delay=5275, delays=5143/0.28/2.8/129, dsn=4.4.1, status=deferred (host relay.cantv.net[200.44.32.36] said: 421 4.4.1 collect: read timeout on connection from mail.domain.com, from=<sender.user@domain.com> (in reply to end of DATA command))

thank for you attention