Virtualmin / Postfix send email issues

We are having issues with our mail server on virtualmin. It has all of a sudden stopped sending emails. We have checked the mail logs and they show as sent but they are not landing in the recipients mailbox if external to our server.

For example if both emails are on our localhost the mail is delivered, but sending an email to external such as gmail is not recieved.

All was working fine yesteday.