• *Many* ISPs use DMARC, not just Yahoo and AOL.​
• The problem is that Yahoo and AOL have set their DMARC configuration so that it tells all DMARC ISPs to reject mail that has a "From:" address from them, but is being sent from someone else.  This part is new.  Please note that other ISPs may do this in the future!
  
• Your Listserv server will distribute posts with a "From:" field of the poster, unless it has been upgraded to level set "16.0-2014a" (build date is 23 April, 2014 for me).
• Then, without the fix,  if the "From:" field is Yahoo or AOL, you have the start of "the DMARC issue", as
• All DMARC ISPs, not just Yahoo and AOL, will reject the distribution. Exactly how each implements "reject" varies.
• Some will drop the distribution into the recipients' spam folder.
• Many will return a "permanent" error suggesting the recipient does not exist.
• Some will return a "permanent" error that tells you (or your list owner) of a DMARC problem.
• If your lists have automatic deletion, sooner or later, Listserv will terminate the subscriptions of everyone that has returned a sufficient number of errors.
• So, not only have your DMARC subscribers failed to receive distribution of posts made by Yahoo and AOL posters, but they will probably lose their subscriptions.
• My lists that had Yahoo and AOL posters lost large numbers of subscribers before we caught and addressed the problem.
• What to do?   Upgrade to the recent Listserv level set asap.  Once you have done that, "the DMARC issue" is behind you, not to bother you or your list owners and subscribers again.
• Until you get Listserv upgraded, you probably want to avoid distributing posts where the "From:" field contains a Yahoo or AOL address.  There are various ways to do this, but you won't like any of them.
  
• How to tell if subscription deletions have taken place or are immanent?  As Valdis suggests, this can be difficult.
• If your lists have change logs, you have a record.   Look at each change log for subscription deletions and the reason, as well as posts from Yahoo or AOL.  If you don't have change logs, why not?
• On Linux, subscribers being monitored are listed in a server file with extension ".autodel".  Have a look; they are almost human readable.  It helps to envision the format of the daily summary of errors a list owner can receive.

Hope this helps!

Cheers, Wayne