Is your feature request related to a problem? Please describe.

Style/SymbolProc's AllowedMethods defaults to [respond_to, define_method] so that you can do

class MyController < ApplicationController
  def create
    respond_to do |format|
      format.json
    end
  end

This could be extended to ActionMailer's.mail method which works similarly:

mail(args) do |format|
  format.mjml
end

Describe the solution you'd like

AllowedMethods defaults to [respond_to, define_method, mail]