Very simple SMTP sinkhole that just prints the messages to stdout

smtp-sink.rb

require 'rubygems'
require 'eventmachine'

class SmtpSink < EM::Protocols::SmtpServer
  def receive_data_chunk( data )
    buffer.concat data
  end

  def receive_message
    puts
    puts "<message>"
    puts buffer.join("\n")
    puts "</message>"
    puts

    clear_buffer!
    true
  end

  def buffer
    @buffer ||= []
  end

  def clear_buffer!
    @buffer = []
  end
end

EM.run {
  EM.start_server( '0.0.0.0', 25, SmtpSink )
}