You didn't say why you wanted to be able to post by email, but I've been wondering the same thing. In my case, I would like to integrate the discussion forum with a listserver and have a post here in the mod requests section:
http://www.phpbb.com/community/viewtopi ... 2&t=586078
My opinion on listservers has changed. I used to hate them and somewhat still do because they are hard to organize and find archive material. However, recent experiences with listservers have made me realize that they are proactive and really help build communities.
I've been using Yahoo Groups recently and I'm very surprised that there are few php listservers out there - maybe majordomo, but looks complicated.
I seem to be alone in my thinking, but it seems like in some cases marrying a list server with a discussion forum would be fantastic.
You might check the 2.xx mods, seems like I've seen post by email discussed before someplace. If you find anything, please let me know.
..... just ran across this - investigating - don't know anything about it .....
"Mail2Forum (or M2F) is an add-on software to the phpBB forum system. M2F combines the functionality of a mailing list system and a phpBB forum in order to add bi-directional 'email to forum' and 'forum to email' communication."
.... also maybe this one: