If you installed Virtualmin, you most likely can’t receive mail from outside your domains. However, you will be able to send mail from the server. Add this to your domains under general settings.
Now your email will be accepted from outside your servers domains to your domains.
If you cannot send mail, you most likely have a host that relays your mail for you. There is a setting that says “Deliver mail directly”. This is where you put your relay host. It may be something like this:
GoDaddy.com is a perfect example if you have them for a host.
Hope this helps.