Postfix Delivery Errors

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

Ref: https://support.google.com/a/answer/2956491?hl=en