This is probably a really simple thing to do, But i can’t find this option.
How to I require that all outgoing email be authenticated with username and password?
This is the only way Virtualmin is ever configured if you used our install script. And, if didn’t use our install script, you shouldn’t be able to send mail at all. 
Anyway, what you’re looking for is called SMTP Authentication, and the service that usually provides it on UNIX and Linux systems is called SASL.
You’ll want to have a look at this old FAQ on the topic:
Ok, Thanks.
One more thing…
I need to authenticate the user using the username@domain.com format. I have followed the directions from here: http://www.virtualmin.com/documentation/id,frequently_asked_questions/#what_s_the_deal_with_in_mailbox_usernames
I changed the default template to use the user@domain.com but It is still requiring a log in with user.domain.
Do I need to change anything else?
thought i should mention that this is a fresh install using the virtualmin install script and migrating my domain from a Plesk8 server.
Ok, I think i figured one thing out.
I created a new email user and it was created with the user@domain.com login and it worked. Unfortunately I have about 500 email users that are displayed as user.domain. Is there a way to change these user in bulk, or do I need to do it one at a time?
Or at least how can I change the existing user that have user.domain over to the user@domain.com format and keep the users email and files intact?
anyone?
Sorry to be impatient but this is the last obstacle before I switch my server over.
Well now it seems I have more issues.
I have done the install from your install.sh script, but the server is not requiring authentication for outgoing email.
Emails are not being delivered to the people’s inbox, even after I deleted their accounts and recreated them (one by one) so they have a user@domain.tld format.
I also can not get the alias domain to deliver emails to the top domains mailbox.
I get this error: <reject@domain1.com> (expanded from <elton@domain2.com>): User unknown in virtual alias table
How long does it take to get support or any answer on this forum or is there another place to get support?
When you say "authentication for outgoing email", how are you (or your users) going about sending email exactly that you want to be authenticated? Are you referring to programs on the server itself sending email, such as webmail?
-Eric
How long does it take to get support or any answer on this forum or is there another place to get support?
It usually takes a few hours. But your questions are tougher (for one reason or another). Users of Virtualmin Professional can file tickets in our support tracker, and expect rapid support–up to and including Jamie or Eric or me logging into your box and correcting whatever problems you’re running into personally. Virtualmin GPL is supported entirely via volunteer effort here in the forums…but it rarely takes more than a few hours to get good answers to good questions.
We generally need to see the relevant log entries, and occasionally information about what configuration you did to get to where you are (permitting relaying is not the default behavior in Virtualmin, so you’ve probably got some custom configuration in Postfix for this to occur).
The troubleshooting common problems guide in the documentation tells you where to find the right log files, and how to resolve some common problems.
What is in your Postfix configuration for the following directives:
smtpd_sasl_auth_enable
smtpd_sasl_security_options
broken_sasl_auth_clients
smtpd_recipient_restrictions
The last one is probably where the trouble lies.