Prevent SMTP facilities for non-hosted domains

[Tourex]

I'm using Plesk Control Panel with Qmail on a centOS platform and provide hosting services for a number of small businesses.

How can I block or prevent users from sending email through their hosted mail account if they try to send mail from an address other than the domain hosted on our server, but using their hosted account details/account to log-in to the SMTP account. We had a case of a rogue employee who used his employers email account to send mailshots using an email address not associated with the client and/or our hosting services.

Thanks in anticipation.