I’m struggling with exactly this issue too. I would really like to be able to get my users posting using WP and bbPress with Postie, rather than mailist, which is pretty archaic.
Here is what I know:
Setting the default content type to topic means that Postie posts the topic without a forum.
Setting the default content type to forum means a new forum is created every time an email is received.
I have not tried to set it as a reply. It looks like without something a little custom, bbPress and Postie cannot be used together to replicate a mailing list type function.
Any suggestions?