How do I modify email headers when using Rules to send mail?
You can still use hook_mail, you just need to identify the key for the rule or rules you want to override. The key is set in the rules_action_mail() function $key = 'rules_action_mail_' . $name . '_' . $element-&gt;elementId(); where name is the rule name and element id is the numeric id for the rule. You need to make sure your custom module has a higher weight that the rules module in the system table of your database.