Checking to see if you received a file that you sent (in LISTSERV) to prevent mail loops is inadequate; you may be external to the actual loop itself. you-->b-->c-->d or some such construct will do it as well (past | | --------- experience with Relay loops). If you could put a message-ID tag (or some similar tag) on a mailfile the *first* time it is received, it could be made to work. Record the x-most recent message ID's received (or that you originated) in a pushdown list. When you receive a file, if it has a tag already, and the tag matches one already in the list, this file has no doubt been here already and should be transferred to the postmaster. You would have to allow this tag to propagate and not strip it like some other tags (don't get me wrong, I *love* getting mail without sixteen sets of 'received by:' headers, but it makes peer mail look curiously 'original').