Zimbra (or postfix for the matter) is not sending outgoing emails. My findings tell me that when an email to email@example.com is to be sent, the DNS query being made returns the direct A records for gmail.com:
gmail.com. 60 IN A 18.104.22.168
gmail.com. 60 IN A 22.214.171.124
gmail.com. 60 IN A 126.96.36.199
And tries to connect to those instead of
dig mx gmail.com:
gmail.com. 2266 IN MX 10 alt1.gmail-smtp-in.l.google.com.
gmail.com. 2266 IN MX 10 alt2.gmail-smtp-in.l.google.com.
gmail.com. 2266 IN MX 50 gsmtp163.google.com.
gmail.com. 2266 IN MX 50 gsmtp183.google.com.
gmail.com. 2266 IN MX 5 gmail-smtp-in.l.google.com.
Ending in time-outs, as expected, since those A records won't be listening on port 25.
I've tried with the DNS Lookup options (MTA tab) fin General Settings and the specific server (is set to use global setting anyway, but I checked) both Enabled and Disabled and the same thing happens.
Any help is welcome.