Get current host and port in mailer

## app/controllers/users_controller.rb
class UsersController < ApplicationController::Base
  def create
    @user = User.new(params[:user])
    if @user.save
      SomeMailer.deliver_activation_email(@user, self) # pass in the controller object
      redirect_to some_path
    end
  end
end

## app/models/some_mailer.rb

class SomeMailer < ActionMailer::Base

  def activation_email(user, controller)
    from "blah@#{detect_host(controller)}"
    recipients user.email
    body    :user => user
    content_type "text/html"
  end

  private

  # from the controller object
  def detect_host(controller)
    @current_host ||= "#{controller.request.host}:#{controller.request.port}"
  end
end

## app/views/some_mailer/activation_email.html.erb

Activate: <%= link_to "Activate", activate_user_url(@user) %>