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 blog to accept new posts from an email (which could even have been
sent from a phone).

See also: https://rubyonrails.org/