dws,
I had a problem not unlike the one you are experiencing with Majordomo once. The way I solved the problem was to send emails from a working client and a non-working client to a local *nix mailbox where I could inspect all the headers. I then telnet'd into port 25 of the server with the lowest weight in the DNS MX record for the domain with the problem. I continued to manually send messages modifying/deleting headers until I found the one that was the culprit. I then modified the code accordingly.

I hope you are able to figure this out without too much trouble.

Cheers - L~R