A dummy mailserver implemented in Ruby, running on the commandline. It just spits out whatever it receives.

dummymail.rb

require 'socket'

class SMTPSession
  @@timeout = 25
  def initialize (session)
    @session = session
    puts "## SESSION STARTED ##"
  end
  
  def write (s)
    @session.write s + "\r\n"
    puts s
  end
  
  def read
    start_timer
    line = @session.gets
    stop_timer
    puts "  " + line
    return line
  end
  
  def close
    @session.close
    puts "## SESSION CLOSED ##"
  end
  
  private
  
  def start_timer
    stop_timer
    @timer = Thread.new do
    sleep @@timeout
    close
    puts "## TIMEOUT ##"
  end        
end

def stop_timer
  @timer.kill if @timer.is_a? Thread        
  end
end


server = TCPServer.new 'localhost', 25
class <<server
  def accept_interruptable
    ts = nil
    Thread.new { ts = self.accept }
    while true
      return ts if ts; sleep 1
    end
  end
end

puts "## Dummymail started ##"

while true
  puts "## WAITING FOR CONNECTION ##"
  session = SMTPSession.new server.accept_interruptable
  begin
    session.write "220 Dummymail ready"
    while line = session.read
      session.write "250 OK"
      break if line =~ /DATA/
    end
    session.write "354 Start mail input; end with <CRLF>.<CRLF>"
    while line = session.read
      break if ".\r\n" == line
    end
    session.write "250 OK"
    while line = session.read
      session.write "250 OK"
      break if line =~ /QUIT/
    end
    #session.write "221 thanks for coming\r\n"
    session.close
    puts "\n\n\n"
  rescue IOError
    next
  end
end