I have given up on this one myself. And many of us have been waiting for the old mail2forum effort, but if you are reading this, you need to also give up on that.
I have had success using Elric's phpbb-mail-gateway
with PHPBB 3.0.11 with YahooGroups. It seems to like email in and email out. It also has the ability to import from maildir path which I am using with the latest grabyahoogroups.pl. This alternative method required me to clean up the import files manually with a perl script, but now I do not have to rely on email, I can easily import the entire archive in the same fashion as it does recent, I can use a single account across multiple groups, and scraping is more secure than email. His script has some great features for those using it with a local listserv, such as auto creating accounts and whatnot, but I use it in a mode where it simply inserts the username without linking and it does not create accounts.
The grabyahoogroups.pl saves each message as a separate file, I duplicated the save lines to make it also save a copy in an import folder which I then operate from. This keeps the original message archive on the computer for historical reference and/or future changes and re-do's.
It is interesting to consider using this as an archive mechanism as well, where every forum post gets emailed out to your own system to be saved as a maildir file. These can easily be put on a web site as a basic and portable archive of your own board history. This post to email function can also be used to synchronize two dissimilar boards if desired. Once you get a feel for tweaking scripts, it really opens the possibility for using them creatively.