[Mailman-Users] Sender address domain expanded - posts rejected by privacy options

[Drew Tenenholz]

Dear fountain of Mailman knowledge --

Something strange is happening on a Mailman 2.1.9 installation that 
is being run for me.  I wonder if it makes sense to any of you out 
there.

We are running a closed list, which severely restricts who is allowed 
to post and subscribe.  This is mostly for an internal discussion 
about a post before it is sent to the announce-only (parallel) list.

One newly added user is having trouble posting.  I believe Mailman 
may be doing something I didn't expect, and we've been able to 
compensate for it, but I'd like to know if I'm going crazy or not. 
I've set up the poster with a series of email account settings as 
follows:

user at subsidiary.org  is an alias on the subsidiary.org mail server 
that simply points to userfirst.userlast at isid.org.  There is no 
mailbox for this user on the subsidiary.org server.  They have a real 
email account on the mail server "isid.org" which they can access via 
both POP and webmail.

When they send mail from the webmail system, their account settings 
for the 'From:' and 'Reply-To:' addresses are user at subsidiary.org.

If I send a message directly to myself from their account, the From 
and Reply-To addresses are correct.

When the message arrives at Mailman 2.1.9, the From address has 
dramatically changed into user at fullMX.record.expanded.edu  What gives!

To be complete, there really is a server at 
fullMX.record.expanded.edu which handles mail (not for this user), 
but I don't know where/why Mailman should be doing some sort of 
lookup and replace the "@subsidiary.org" with the full MX name.  So, 
we updated the Privacy>Sender Filters (and General>Moderators) had to 
include this address and everything is working.

The real question is why did this happen?  Is it expected?  Has it 
been changed in later versions?  If there is a FAQ or something to 
tell me how this works, I'd love to read it.

Thanks in Advance,
Drew Tenenholz