Action Mailer is a framework for designing email-service layers. These layers
are used to consolidate code for sending out forgotten passwords, welcome
wishes on signup, invoices for billing, and any other use case that requires
a written notification to either a person or another system.

Action Mailer is in essence a wrapper around Action Controller and the
Mail gem.  It provides a way to make emails using templates in the same
way that Action Controller renders views using templates.

Additionally, an Action Mailer class can be used to process incoming email,
such as allowing a weblog to accept new posts from an email (which could even
have been sent from a phone).

This is for Ruby on Rails 5.2.