MTA Settings for Ruby

The mta-settings gem enables transparent MTA (mail transport agent) configuration from the environment for both ActionMailer and Mail, based on either an explicit MTA_URL variable or popular conventions for Sendgrid, Mandrill, Postmark, and Mailgun (as provided by Heroku addons, for example).

Add this line to your application's Gemfile:

gem 'mta-settings'

If mta-settings is required (which Bundler does automatically by default), ActionMailer configuration is fully automatic. With Rails, be aware that config.action_mailer assignments will take precedence, so you might want to strip those out of your apps config/environments/ files.

Mail.defaults do
  delivery_method *MtaSettings.from_env
  # delivery_method *MtaSettings.from_url(ENV['MTA_URL'])
end

Configuration will happen based on the presence of the following environment variables, in order of decreasing precedence:

• MTA_PROVIDER: points to another environment variable containing an MTA URL
• MTA_URL: See below
• SENDGRID_USERNAME: Sendgrid
• MANDRILL_APIKEY: Mandrill
• POSTMARK_API_TOKEN: Postmark
• MAILGUN_SMTP_LOGIN: Mailgun
• MAILTRAP_API_TOKEN: Mailtrap (for development)

If no supported environment variable is found, the configuration is left blank. This enables easy defaulting:

ActionMailer::Base.delivery_method ||= :letter_opener

The scheme of an MTA URL is used to set the delivery method. The user, password, host, port, and path portions are used to populate the user_name, address, port, and location settings of the chosen delivery method. Query parameters are then merged in.

• The sendmail and file adapters both respect the ActionMailer location defaults, so you can just give sendmail:/// or file:///.
• If a path is given in an smtp URL, it will be used as domain rather than location (minus the leading slash).
• If domain is set, the default from address will be set to noreply at that domain.

Here's an example for Gmail:

smtp://username%40gmail.com:password@smtp.gmail.com:587/

Using an MTA URL is highly recommended even when your SMTP provider is supported out of the box. MTA URLs are much easier to copy between environments or try out locally for debugging.