you need to generate views for the mails that are sent to the user
rails g mailboxer:views
this will generate 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 those templates .
Another thing that you can do is to uncomment/add these lines in /config/initializer/mailboxer.rb
config.notification_mailer = CustomNotificationMailer
config.message_mailer = CustomMessageMailer
and create above two mailers classes which must contain the following function
send_email
the parameters to this function and their implementation is given on this link . You will get a hint of how to write your own send_mail function .
Notification Mailer Send Email function
Message Mailer Send Email function