Communicating/mailing through Drupal site

I wonder what is the best way to handle the following scenario:

1) User A fills out webform
2) Webform content is sent to user B from [email protected]
3) User B replies to [email protected]
4) Site sends user A the email content from [email protected]
Continue if necessary: B replies, A replies,…

The whole communication should be available to both users on the site too. To ensure the site knows about the communication, the mail addresses of the users should not be exposed.

Thanks for any hints, tips, suggestions,…

Drupal version: