How to stop sending emails for all servers and users

[calport]

@post

NO, i mean if the user use the SMTP in another Email Client.

Yes, the users of all virtual servers will not be able to send out email to the internet using your SMTP server even if they use an email client.

Also see:

Disable Mail for Domain Does Not Prevent Mail being sent from Wordpress

In addition to all the other suggestions posted above, if you use Virtualmin’s mail limits feature (Virtualmin → Email Settings → Mail Rate Limiting - you really should if you don’t) then set it to zero for the domain on which you don’t want WordPress to send outgoing email. Would that not work for you? PS You might have to enable mail for the virtual server to be able to use Virtualmin’s mail limit feature.