This is already possible with available tools, for free or close-to-free.
1) You need a server to act as the incoming gateway, so that incoming mail is isolated from locally-originated mail. SmarterMail Free edition works well for this purpose. If your main installation is supported, SmarterMail will answer support questions that apply to the incoming gateway as well.
2) Integrate Declude from MailsBestFriend into your incoming gateway. Among other skills, it can add your choice of a Subject prefix, a body header, or a body footer. Declude is also free, at least for now. You should also buy their modestly-priced support services to build a relationship. If you are currently using only the built-in spam filtering within SmarterMail, you should check out the commercial products that they offer as well.
3) Build better mail filters into your email defenses, so that the spoofed stuff does not get through in the first place. Declude does a lot of things well, but its most important attribute is that it is customizable with programs or scripts that you create. I have customized it extensively. Between its built-in capabilities and the customization, I am able to control my mail flow in ways that could never be achieved with the commercial appliances that we have used for 15 years. I still use the commercial spam filter because I cannot reproduce its capabilities for content filtering. But that appliance only looks at the stuff that Declude has not discarded.