[sndstrm]

I already coded a private bbPress Plugin, that extends it to something like Mailman.
It just periodically catches mails to @forum.domain.tld via IMAP and adds a new reply to the topic with the Mail content (quotation skipped).
The notification is tweaked: the Reply-To header is replaced by reply-[topic-id]@forum.domain.tld, so the user can easily reply to the topic. But this is currently done by completely replacing the bbp_notify_topic_subscribers function with my own one (including the original content, just replacing the Reply-To).
All calls to bbp_subscription_mail_headers and bbp_subscription_from_email or bbp_subscription_to_email are just called with $headers or the address itself. As of 1.5.6 the Reply-To is no longer set by bbPress.

Will there be a way to include the topic-id (preferred in header altering) in upcoming releases? Or maybe a solution by bbPress itself to respond to notification Mails?

[Robkk]

i say look how this plugin is developed and see if you could learn a thing about adding what you want to yours.

https://wordpress.org/plugins/asyncronous-bbpress-subscriptions/