For those on the hunt, I have built a functioning solution to this using Elric's Mail Gateway
. It works out of the box for pop input and smtp output. The script also has capacity to look in maildir folder for import and that's the way I decided to go. I am using grabyahoogroups.pl to scrape new messages, I have the script saving a second copy of each message in an IMPORT folder to work from while keeping the original as an archive, I wrote a perl script to heavily clean up the files, and then I execute Elric's importer. All of this runs automatically on a Windows server.
If you have been using yahoo2mbox.pl like I have over the years, you can easily find a perl script to separate out into individual messages, so your entire archive can be imported. It can be done, BUT!, you will find that as of 2013, Yahoo purposely screws up the From: header which kills a lot of email processing scripts. Also, you have to clean out anything that is not just text, which means cleansing or cutting off html, multi-part and other crud before trying an import. You'll see what I mean.
I highly recommend working with something new and encompassing like this on a development board, do not use an active board until you have all the bugs ironed out. I've had mine running for a few weeks and I still am finding bugs that I need to add to the cleansing script I made.
I am not a programmer. I picked up some perl while doing this, and if you are not willing to do that, you will have to use another language or just not succeed. But, Elric's script will operate with very little effort in the pop-in mode without all of this hassle. So if you just want to "connect" to a list, this is your script. If you want to import a scraped archive and/or operate with maildir/mbox files, this wont be simplistic, but it is the only solution I have found since mail2forum died its horrible drawn-out death.