wild card modal email forwarding to rails application

Hi
  May be a long story. Sorry for that.My requirement as this. I have a
saas application. The different people can create accounts. Each account
holder can create users under them . An example is during account
creation one user gives email as "admin1@mydomain.com'. Then after login
to application he creates say two users and gives (valid) emails as
user1@gmail.com and user2@yahoo.com.

  At a later stage from the application he send mail to both the above
users and they get mail in there gmail and yahoo inboxes. Also what is
sent to them etc are stored to db also as a thread. Upto this everything
works fine.

   So coming to my requirement suppose user1@gmail.com replies to this
email. Then admin1@mydomain.com gets email in his inbox(This also can be
done). And also it has to be reached to my rails application . Dont know
how to do this???.And also to be logged to my db.

    The above is the original requirement. As part of that what I would
like to know is can I reply mail to user_thread_id@mydomain.com (An
example is user_100@mydomain.com) . The thing is user_100@mydomain.com
is not a valid email. Like that there may be user_2@myexample.com
,user_5@myexample.com All these are dynamically created when replying.
        And what I need is, all these mails has to be reached at
mydomain.com and forwarded to my rails application. From there I can get
the thread_id from this mail (example 100 from user_1--@mydomain.com)
and also the mail contents so that everything like subject,body etc can
be stored to my db.

   Dont know I could make you understand my requirement. Please tell if
you did not understand. Guide me with your valuable comments

Thanks