On April 4th, 2014 Yahoo updated the DMARC record with "p=reject" for the "yahoo.com" domain. This was done to proactively by Yahoo to protect their users from increasing email spam that uses Yahoo email addresses from other mail servers. On April 22nd AOL similarly updated it's DMARC record with "p=reject" for the "aol.com"
All DMARC compliant mail receivers (including Yahoo, Hotmail, and Gmail) are now bouncing emails sent as "@yahoo.com" and "@aol.com" addresses that aren't sent through Yahoo servers or AOL servers. Any messages without a proper Domain Keys Identified Mail (DKIM) signature or Sender Policy Framework (SPF) alignment will be rejected.
How this impacts Listserv:
A subscriber/list member from *@yahoo.com or *@aol.com posts a message (or a replies to a message) to your list. This will go to some subscribers, but will NOT go to subscribers/list members at yahoo.com. Messages to this domain will not only not be delivered but will be bounced/refused as if the recipients at those domains do not exist. Thus, after a few such messages, your subscribers/list members at all of those problematic domains are at risk to be auto-deleted from your list(s).
At this point there is no easy resolution to this issue. L-Soft (the vendor that created Listserv) is working on a patch to correct the issue, but it won't be available soon, as it will be a server upgrade, so it has to be tested before it's released.
For now, you must prevent subscribers/list members from *@yahoo.com or *@aol.com addresses from posting any messages to your list. They simply cannot participate in your list if they send from a *@yahoo.com or *@aol.com address. They can receive and read messages from other subscribers/list members and they can send from another address, if they have one.
The LISTSERV command is: QUIET SET listname NOPOST FOR *@yahoo.com* or QUIET SET listname NOPOST FOR *@aol.com*
Send a message to your list explaining the issue and what action members from *@yahoo.com and *@aol.com addresses will need to take to continue to participate in the list. (Basically, get a new email address.)
ALTERNATIVE: You can set all members from *@yahoo.com and *@aol.com addresses to REVIEW status. This means their messages will come to a moderator/editor of your list. The moderator/editor can then forward/repost the message so that it comes from their own address, not from the *@yahoo.com or *@aol.com address that originally posted the message. If your list has no moderator/editor the list owner will receive the message. This can be a lot of work and for some lists the traffic is simply too great for this to be a viable option.
The LISTSERV command is: QUIET SET listname REVIEW FOR *@yahoo.com* or QUIET SET listname REVIEW FOR *@aol.com*
These commands can be issued from the Listserv web site by navigating to List Management > LISTSERV Command, type in the "Command:" field, then click the "Enter Command" button.
Other actions you can consider:
Create a filter for yahoo accounts, which prevent posted messages, and new subscribers from posting messages or joining your list.
List configuration setting: Filter= also, *@yahoo.com* or Filter= also, *@aol.com*