you need to create views for letters sent to the user,
rails g mailboxer:views
this will create two different folders in<your rails app>/app
1- notification_mailer - containing template files when a notification is sent to a user
2- message_mailer - containing template files when a message is sent to a user
You can change these templates.
Another thing you can do is uncomment / add these lines to /config/initializer/mailboxer.rb
config.notification_mailer = CustomNotificationMailer
config.message_mailer = CustomMessageMailer
and create above two classes of mailers that should contain the following function
send_email
The parameters of this function and their implementation are given at this link. You will get an idea of how to write your own send_mail function.
Mailer
Message Mailer