Up to my knowledge, There is nothing ready to use for forums, but it is not unlikely, that several have implemented this based on the available infrastructure. The acs-mail-lite (part of acs-core) has some functionality to process incoming mails (e.g. mail bounces) and provides means to extend mail processing via callbacks and a ReplyPrefix. Arbitrary application packages can provide callback to handle the parsed mails. If you are interested, you can check in the notification package or in the bug-tracker for examples of such callbacks
A short online description is here:
https://openacs.org/api-doc/proc-view?proc=acs_mail_lite%3a%3aload_mails
