RFC 2392 Message-ID values are supposed to be unique, but they may not be if the software generating them is buggy. I remember reading somewhere about some emai

RFC 2392 Message-ID values are supposed to be unique, but they may not be if the software generating them is buggy. I remember reading somewhere about some email program that assumed they were unique (in effect using them as a natural key). If you were unlucky enough to get messages with identical Message-ID values, all but one would disappear.