Here you can see how to set parameters for the great Sendgrid's SMTP API (Take a look to lines 15-19) with Ruby on Rails

mailer.rb

class ReportMailer < ApplicationMailer

  def daily_email(report)
    @report = report
    @user   = report.user
    
    headers["X-SMTPAPI"] = { 
      category:    report.category, 
      unique_args: { report: report.id, user: report.user.id }
    }.to_json

    mail(
      to: report.recipients, 
      subject: report.name,
      from: 'Dailytics.com <[email protected]>'
    )
  end
end