marker for unique emails?

marker for unique emails? Hi Rebols is there something in the email-header which is guaranted to be unique. message-id ? i want to avoid double emails, like when i download my mails multiple times while testing, without removing them from the server. now i look for a way to identify the doubles without much overhead, so i want to cache a reliable "checksum" in a file. currently i use checksum , skipping first two lines (they change, "received" with changing date). after that i double-check with my archive if equal. which may work only with my provider? -Volker