gistfile1.rb

# Base class
class DynamicHostMailer < ActionMailer::Base
	def initialize method_name=nil, host, *args
		@host = host
		super method_name, *args
	end

	def url_options
		# Host should be checked against a relevant whitelist, this is only an example
		{ host: @host }
	end
end

# Mailer
class SomeMailer < DynamicHostMailer
	def my_mail user
		mail to: user.email, from: "[email protected]", subject: "This is an email"
	end
end

# Sending
SomeMailer.my_mail request.host_with_port, user