I have a website running a social network Buddypress.

Users leave comments on other users "walls". What I would like to do is to allow users to reply to each other inside the email client, without logging in Buddypress.

My idea was this one: each comment generates an email notification, and the notification has a Reply-To field filled with a long unique identifier (UUID). I would keep in a table the tuples (UUID, messageId, notifiedUser)

On the server side, when I receive an email in, say, Postfix, I would parse the recipient to see if there is a previously generated UUID, and check if the emitter is the person which the notification was sent to. If I find a match, I parse the email body in search of the reply, then I call some buddyPress PHP code to insert the text inside the database.

My question is: is it secure, and what attacks is this vulnerable to ?

Clearly, somebody sniffing (UUID, recipient) could immediately send garbage by impersonating the emitter (i.e sending an email with a forged From: field, equal to the To: field of the email he just sniffed). But is it a real concern ? Which paths used by emails are encrypted ?