AOL and Yahoo! now use DMARC to force email services like SendGrid and MailChimp to reject emails if the FROM field gets 'spoofed' for their domain. To get around this, we will check for the `p=reject` key/value combo in the TXT record on _dmarc.domain.com. Here is a quick and dirty way to check if the DMARC field is set using ActionMailer on Ru…

action_mailer.rb

class ActionMailer::Base

  def dmarc(email)
    domain = email.split('@')[1]
    dmarc_domain = "_dmarc.#{domain}"
    
    Resolv::DNS.open do |dns|
      records = dns.getresources(dmarc_domain, Resolv::DNS::Resource::IN::TXT)
      records.empty? ? nil : records.map(&:data).join(" ")
    end
  end

  def dmarc?(email, key, value)
    key = key.to_s
    value = value.to_s

    record = dmarc email
    return false if record.blank?

    record_items = record.split(';').map(&:strip)

    hash = {}
    record_items.each do |item|
      k,v = item.split('=')
      hash[k] = v
    end

    hash[key] == value
  end

end

support_emails_controller.rb

class SupportEmailsController < ApplicationController

  def create
    @support_email = SupportEmail.new post_params
    if @support_email.save
      SupportMailer.regular(@support_email).deliver
      flash[:notice] = "Thanks for contacting us! We'll get back to you as soon as possible."
    else
      flash[:alert] = "There was a problem sending. Please email us directly"
    end
    redirect_to support_path
  end


protected

  def post_params
    safe_params = [
      :from, :body, :subject
    ]
    params.require(:support_email).permit(*safe_params)
  end

end

support_mailer.rb

class  SupportMailer < ActionMailer::Base
  default :to   => "support@mydomain.com"
  default :from => "support@mydomain.com"

  def regular(support_email)

    @support_email = support_email

    options = {
      subject: support_email.subject
    }

    if dmarc?(support_email.from, :p, :reject)
      options[:reply_to] = support_email.from
    else
      options[:from] = support_email.from
    end

    @mail = mail( options ) do |format|
      format.text
      format.html { render :layout => false }
    end
  end

end