[Mailman-Users] strange bounce message

[Anne Ramey]

A couple of people have sent replies to one of my lists today and 
several of the messages bounced like this:

       ===============================================
 >               THIS IS A WARNING MESSAGE ONLY
 >           YOU DO NOT NEED TO RESEND YOUR MESSAGE
 >       ===============================================
 >
 > A temporary error occured while delivering to the following address(es):
 >
 >   <pfenno at ncees.org>: 454 TLS not available, must be connected via TCP
 >
 > I will continue trying to send the message until it is delivered or
 > expires.

They've replied without incident before.  It looks like they have 
changed something on their end to require the message to be delivered 
TLS, which the server this message is attempting to deliver to does not 
support, and that there is nothing I can do...is that correct.  What can 
I tell the sender to do to prevent this? Any pointers would be 
appreciated.  I've been using mailman for a couple of years and haven't 
run into this before.
-- 
Anne