Action Mailbox provides inbound email processing for features like "reply by email".
You can run rails action_mailbox:install to install Action Mailbox in your application. Don't forget to run rails db:migrate afterwards to run the migrations
You can run rails g mailbox my_mailbox to create a new Action Mailbox...mailbox. This will generate the Ruby class for processing inbound emails.
Check out the GoRails screencast episodes on Action Mailbox and the Action Mailbox Rails Guide for more information on how to use Action Mailbox.