postfix allow senders

I’ve a virtualmin installed on a CentOS 6.5, this server hosts a lot of domains of my customers. Recently I noticed an increasing number of attacks for sending spam in one of my host, and my customer doesn’t know where to patch his code for fixing the bug.

Looking at the queues of postfix I noticed that senders of these emails are so I’d like to allow access to postfix only to specific users like,, and so on…

Is there something similar in virtualmin?

Thank you

Disable mail in PHP and allow only SMTP auth. with PHP
Disallow unauthenticated email from localhost.

Probably some of your clients were hacked and if this is true you have bigger problem to solve. Depending what is your deal with them will force you to some action, from disabling that account to cleaning infected website. Either way just disabling email to some clients and pretend everything is ok is not right solution.