InternetTechnically speaking, the unique reply emails are most likely used to handle bounced emails. The server will parse the bounced email and simply remove, or alert QF to the email address that didn't work. This is often used in mailing list management software and has been around for years. It can also be used to track who is

a) replying to emails
b) opening emails, if using html

This technique was used by spammers in 2001-2002.
Hope this helps!