Module can included in a mailer to help warmup a new email provider on a static IP. This code was written for a transition from sendgrid to mailgun for a mailer which handled about 200k emails/day. The constants are set to transition from 100k/day to 3m/day with a 25% daily increase.

warmup_email_provider.rb

module mail_provider_warmup
  DAILY_INCREASE_RATE = 1.25 # 25%
  INITIAL_MAIL_VOLUME = 100000
  FINAL_MAIL_TARGET = 3000000.0
  WARMUP_START_DATE = Date.new(2014,7,24)
  PROVIDER_FROM = :sendgrid
  PROVIDER_TO = :mailgun
  
  #selected_provider = self.class.select_provider(rand)
  def self.select_provider(random)
    return :PROVIDER_FROM if Date.today < WARMUP_START_DATE
    random < provider_probability ? PROVIDER_FROM : PROVIDER_TO
  end
  
  def self.provider_probability
    current_daily_mail_volume(days_since_warmup_start).to_f / FINAL_MAIL_TARGET
  end
  
  def self.current_daily_mail_volume(day)
    return INITIAL_MAIL_VOLUME if day < 0
    INITIAL_MAIL_VOLUME * (DAILY_INCREASE_RATE ** day)
  end
  
  def self.days_since_warmup_start
    (Date.today.to_date - WARMUP_START_DATE.to_date).to_i
  end
end

The expectation is that your application has implemented a provider input which can be used to set up your mailer. Read this article from the RailsApps team to learn more about the options you have for email service providers: http://railsapps.github.io/rails-send-email.html.

Sendgrid has good documentation about warming up an IP and why it is important: https://sendgrid.com/docs/User_Guide/warming_up.html.