php mail() sending emails not related with domain

You can remove sendmail and only allow SMTP authentication from PHP scripts. sendmail doesn't have the capabilities of exim for configuration and it's neigh impossible to prevent spoofing and spamming using it. Joomla specifically has the option to use SMTP authentication to send emails in the configuration settings.

Yes, some users would have to modify their scripts who have not set them to use SMTP authentication if sendmail is no longer on the system, but that's the price to pay for increased security.

Now, if you do enable SMTP authentication only and want to prevent spoofing using it, you can put the following ACL into the exim configuration:

Code:

acl_check_data:
deny
 authenticated	 = *
 condition = ${if or {{ !eqi{$authenticated_id} {$sender_address} }\
  { !eqi{$authenticated_id} {${address:$header_From:}} }\
 }\
 }
 message	 = Your FROM must be as the account you have authenticated with, your email is not delivered.

This would go into the box where it has begin acl directly about it (the second box in the WHM > Exim Configuration > Advanced Editor area) and should prevent spoofing via webmail and anyone authenticating with SMTP.

I actually mention the above rule along with some other tips to help with tracking spammers in this forum post:

http://forums.cpanel.net/f43/open-relay-170798-p2.html#post777302