Accept reply emails only

what is the most easiest way of implementing this?

I want it so that my mail server only accepts email that one of the users of my site sent first. This can be done by me assigning a code attached to the email, and store this code on a MySQL table. Easy.

But what is the best method to store this code with emails? In the email its self, in the subject and put a note on the email not to change the subject? Is there a header I can send to the recepicant so that when they do reply the same header is sent back?

Regards.

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