If your using Gmails SMTP relay and receive either of the following errors, switch your outbound mail to use port 587.
Show Postfix errors; tail -f /var/log/syslog
Look for either of these;delivery temporarily suspended: connect to smtp-relay.gmail.com
delivery temporarily suspended: lost connection with smtp-relay.gmail.com
The fix;vi /etc/postfix/main.cf
Update the realyhost;relayhost = smtp-relay.gmail.com:587