If you want to make your Rails application be capable of sending SMTP emails, check out the action mailer configuration section Show archive.org snapshot in the Ruby on Rails guide.

TL;DR you will end up having an smtp_settings hash that looks something like this:

smtp_settings = {
  address: ...,
  domain: ...,
  port: ...,
  user_name: ...,
  password: ...,
  authentication: ...,
  tls: ...,
  enable_starttls_auto: ...,
}

This hash can be set as the delivery_method in your environment or individually applied to an email generated in your mailer:

# production.rb:
Rails.application.configure do
  config.action_mailer.delivery_method = :smtp
  config.action_mailer.smtp_settings = smtp_settings
end

# test_mailer.rb
def test_mail
  mail(to: 'mail@example.com', subject: 'Subject').delivery_method.settings.merge!(smtp_settings)
end

Configuring SMTP with a Google account

These is an example configuration you can use to test if mail delivery works in your application:

smtp_settings {
  address: 'smtp.gmail.com',
  domain: 'smtp.gmail.com',
  port: 587,
  user_name: 'your_email@google.com',
  password: 'google_app_password',
  authentication: 'login',
  tls: false,
  enable_starttls_auto: true,
}

If you are using Google with 2FA, you still can deliver mails

The solution for SMTP mail delivery from a google account with two factor authentication is described here Show archive.org snapshot . You basically create a password that bypasses your security efforts. So make sure that you DONT GIVE IT AWAY, Google also suggests you to not write it down, etc.