Barry Warsaw writes:
Second, things can happen to a list
that might cause this sequence number to get corrupted.
Add an X-Mailman-Sequence-Number header if not already present.
That doesn't deal with your other comments, but as I point out elsewhere, if you don't use *any* Mailman-specific information in the global ID, you have no sane way to handle collisions except throw them away (or make the global ID refer to a collection resource, but that's kinda unintuitive).