is this even possible?

I have been asked to work on a forum using drupal as a base and there is this one requirement that has me stumped.

:- Users must be able to reply to subscribed topics by sending an email to the system which then posts it in the right thread for them. You must also be able to send the data in that email to the person who started the thread.

Can this be done with a drupal system?

and if so which modules does this require?