For Listserv on UNIX or Windows, you must be at least to version "LISTSERV 16.0 - 2014a" and preferably to "LISTSERV 16.0 -2014b" before you allow AOL & Yahoo addresses, and others using DMARC such as AOL/Yahoo, to post.

If you don't, when (for example) AOL receives the mail for your AOL subscriber(s), the AOL mail system sees that it was "From:" an AOL address (your AOL poster), but not sent from an AOL server (it was sent from your Listserv server).  This results in an internet standard "permanent" bounce message and non-delivery.  Listserv sees the permanent error and, depending on your list settings, terminates the subscription(s).

There are a number temporary workarounds to protect your AOL/Yahoo! subscribers.  None will be to your liking or the likes of your AOL/Yahoo people.

The fix, and it solves the problem, is to upgrade to "Listserv 16.0 2014b".  At that point, your AOL and Yahoo! addresses will be able to post with no bad effects on your AOL/Yahoo! subscribers.