**PLEASE DO NOT REPLY TO THE LIST ON THIS MATTER.**
Following a query by an aol user who kept getting unsubscribed, I did some digging, and the list has a problem with respect to mail from yahoo/ hotmail/aol, and a few other smaller domains.
As far as I can tell, as summarised at http://postmaster-blog.aol.com/2014/04/22/aol-mail-updates-dmarc-policy-to-r... , a few fairly large email providers have unilaterally implemented a policy called DMARC, and are now rejecting inbound emails that don't meet the policy requirements. In brief, the requirements are based on the content of the From: (the Header-From, rather than the Envelope-From) header line, and basically amount to "no mail may be sent with a From: line that matches our domain, from outside our domain).
Until recently, this meant that whenever a list user from an afflicted domain sent mail to the list, noone inside an afflicted domain would receive it, and a bouncee would be generated for each such user. Recently, list traffic went up enough that sufficient bounces were generated to unsubscribe everyone on the list who was subscribed from an afflicted domain (47 users, the vast majority from yahoo and hotmail).
They can resubscribe, and some have, but they will still not see emails from users in DMARC'ed domains, and if the list has another busy period, they'll all be unsubscribed-by-bounce again.
As far as I can tell, the only remedy is to reconfig the list so that the From: address is set to a generic address, probably toasters@teaparty.net, rather than the actual sender (as it is now). This will mean that misconfigured autoresponders will send to everyone on the list, rather than to the sender as they do now. Before you ask, no, I am not willing to set it my own address, and sort through all the bounces manually.
I've polled the other lists I'm on, and most (90%) of them seem to be ignoring the problem. I think DMARC is a rather foolish attempt to make the contents of the From: header reliable (at least as regards domain name), and am therefore inclined to ignore it, and say to yahoo/AOL/hotmail users that they need to subscribe from a provider with a bit more clue. But as usual I will be guided by you lot in this.
If anyone feels strongly that the list should be reconfigured as described above, please let me know in the next week or two. If you have strong feelings the other way, it would be useful for me to know that also. And if anyone knows more about this than I do, and is aware of errors in the above, I'd value an off-list email even more.