require 'net/smtp'

message = <<MESSAGE_END
From: Sender <name@sender_domain.com>
To: Receiver <name@receiver_domain.com>
Subject: SMTP e-mail test

This is a test e-mail message.
MESSAGE_END

Net::SMTP.start('mail.smtp.com', 25, 'mail.smtp.com', 'username', 'password', :login) do |smtp|
  smtp.send_message message, 'name@sender_domain.com', 
                             'name@sender_domain.com'
end