[Mailman-Users] undelivered mail, but only to some

[D. Alan Stewart]

I am having a baffling problem with Mailman. Suddenly the list that I  
am administrating has stopped delivering mail to a subset of the  
subscribers. It's not readily apparent what those that are not  
receiving the mails have in common - the domains that I know of  
include mac.com, yahoo.com, av8inc.com. It also seems that anyone new  
added to the list does not receive emails, though they do receive the  
initial subscripion notice and the monthly reminders. I have seen  
once that someone newly subscribed received one posting and has not  
received another since.

This list is provided to me by my web hosting provider, who is also  
baffled. The Mailman installation is version 2.1.5. The sendmail logs  
show successful transmissions to the users in question, at the  
correct time, so it appears Mailman is sending something, but the  
receiving end  must be discarding it. I'm one of those not receiving  
the emails. I've contacted my mail provider, mac.com, but they want a  
copy of the email being sent with complete headers. Since any new  
subscribers I've added aren't getting emails I'm having trouble doing  
this.

We set up a test list. I've found that the test list works if I  
subscribe only myself. However, if I mass subscribe all the  
subscribers of the malfunctioning list to the test list, it appears  
to behave in the same manner as the malfunctioning list. That seems  
to imply that it is either the number of subscribers or that one bad  
entry in the list is creating the problem. It can't be the number of  
subscribers, there are other, bigger Mailman lists on this same host  
that seem to be working correctly. There are less than 100  
subscribers to my list. In examining the list that I am subscribing I  
can find no entries that appear malformed.

Does anyone have any suggestions?

TIA.
Alan