class WelcomeEmail < ApplicationMailer

    def welcome_email(user)
        @user = user
        mail to: @user.email, subject: "Thanks for joining!", from: "info@app.com"
    end

end

class User < ApplicationRecord
    devise :database_authenticatable, :registerable,
           :recoverable, :rememberable, :validatable, :confirmable

    def after_confirmation
        WelcomeEmail.welcome_email(self).deliver
    end

end

<h1>This is the welcome email!</h1>

Rails.application.configure do

  config.action_mailer.perform_deliveries = true
  config.action_mailer.delivery_method = :smtp
  config.action_mailer.smtp_settings = { :address => "localhost", :port => 1025 }
  config.action_mailer.default_url_options = { host: 'localhost', port: 3000 }

end

Rails.application.configure do
  # for emails to go out
  config.action_mailer.default_url_options = { host: 'appname.herokuapp.com', protocol: 'https' }
  config.action_mailer.delivery_method = :smtp
  config.action_mailer.perform_deliveries = true
  config.action_mailer.smtp_settings = {
    :user_name => ENV['SENDGRID_USERNAME'],
    :password => ENV['SENDGRID_PASSWORD'],
    :domain => 'herokuapp.com',
    :address => 'smtp.sendgrid.net',
    :port => 587,
    :authentication => :plain,
    :enable_starttls_auto => true
  }
end