A 77 line SMTP server, and a 253 line POP3 server.

1_smtpd.rb

require 'gserver'
require 'rubygems'  
require 'active_record'  

dbconfig = YAML::load_file(File.dirname(__FILE__) + '/config/database.yml')
ActiveRecord::Base.establish_connection(dbconfig['development']) 

# CREATE TABLE emails (id integer primary key autoincrement, mail_from, rcpt_to, subject, email, user_id integer);
# CREATE TABLE users (id integer primary key autoincrement, username, password, email);

class Email < ActiveRecord::Base   
end 

class User < ActiveRecord::Base   
end

class SMTPServer < GServer
  def serve(io)
    @data_mode = false
    @email_message = ""
    puts "Connected"
    io.print "220 hello\r\n"
    loop do
      if IO.select([io], nil, nil, 0.1)
        data = io.readpartial(4096)
        puts ">>" + data
        @email_message << data
        ok, op = process_line(data)
        break unless ok
        puts "<<" + op
        io.print op
      end
      break if io.closed?
    end
    db_insert(@email_message)
    io.print "221 bye\r\n"
    io.close
  end

  def process_line(line)
    if (@data_mode) && (line.chomp =~ /^\.$/)
      @data_mode = false
      return true, "220 OK\r\n"
    elsif @data_mode
      @email_body += line
      return true, ""
    elsif (line =~ /^(HELO|EHLO)/)
      return true, "220 and..?\r\n"
    elsif (line =~ /^QUIT/)
      return false, "bye\r\n"
    elsif (line =~ /^MAIL FROM\:/)
      @mail_from = (/^MAIL FROM\:<(.+)>.*$/).match(line)[1]
      return true, "220 OK\r\n"
    elsif (line =~ /^RCPT TO\:/)
      @rcpt_to = (/^RCPT TO\:<(.+)>.*$/).match(line)[1]
      return true, "220 OK\r\n"
    elsif (line =~ /^DATA/)
      @data_mode = true
      @email_body = ''
      return true, "354 Enter message, ending with \".\" on a line by itself\r\n"
    else
      return true, "500 ERROR\r\n"
    end
  end
  
  def db_insert(email)
    subject = (/^Subject\: (.+)$/).match(@email_body)[1]
    u = User.find(:first, :conditions => { :email => @rcpt_to })
    if u and @mail_from and @rcpt_to
      Email.create(:mail_from => @mail_from, :rcpt_to => @rcpt_to, :subject => subject, :email => @email_body, :user_id => u.id)
    end
  end
end

a = SMTPServer.new(2225)
a.start
a.join

2_pop3d.rb

require 'gserver'
require 'digest/md5'
require 'rubygems'  
require 'active_record'  

dbconfig = YAML::load_file(File.dirname(__FILE__) + '/config/database.yml')
ActiveRecord::Base.establish_connection(dbconfig['development']) 

# CREATE TABLE emails (id integer primary key autoincrement, mail_from, rcpt_to, subject, email, user_id integer);
# CREATE TABLE users (id integer primary key autoincrement, username, password, email);

class Email < ActiveRecord::Base
  def initialize(*kvars)
    @deleted = false
    super(kvars)
  end
  def deleted?
    @deleted
  end
  def deleted=(deleted)
    @deleted = deleted
  end
end 

class User < ActiveRecord::Base   
end

class POP3Server < GServer
  attr_writer :hostname
  def serve(io)
    @state = 'auth'
    @failed = 0
    @apop_challenge = "<#{rand(10**4 - 1)}.#{rand(10**9 - 1)}@#{@hostname}>"
    io.print("+OK POP3 server ready #{@apop_challenge}\r\n")
    loop do
      if IO.select([io], nil, nil, 0.1)
        begin
          data = io.readpartial(4096)
          puts ">> #{data.chomp}"
          ok, op = process_line(data)
          break unless ok
          puts "<< #{op.chomp}"
          io.print op
        rescue Exception
        end
      end
      break if io.closed?
    end
    io.close unless io.closed?
  end
  
  def user(username)
    @user = User.find(:first, :conditions => { :username => username })
  end
  def pass(password)
    return false unless @user
    return false unless @user.password == password
    true
  end
  def emails
    @emails = [ Email.find_all_by_user_id(@user.id) ].flatten
  end
  def stat
    msgs = bytes = 0
    @emails.each do |e|
      next if e.deleted?
      msgs += 1
      bytes += e.email.length
    end
    return msgs, bytes
  end
  def list(msgid = nil)
    msgid = msgid.to_i if msgid
    if msgid
      return false if msgid > @emails.length or @emails[msgid-1].deleted?
      return [ [msgid, @emails[msgid].email.length] ]
    else
      msgs = []
      @emails.each_with_index do |e,i|
        msgs << [ i + 1, e.email.length ]
      end
      msgs
    end
  end
  def retr(msgid)
    msgid = msgid.to_i
    return false if msgid > @emails.length or @emails[msgid-1].deleted?
    @emails[msgid-1].email
  end
  def dele(msgid)
    msgid = msgid.to_i
    return false if msgid > @emails.length
    @emails[msgid-1].deleted = true
  end
  def rset
    @emails.each do |e|
      e.deleted = false
    end
  end
  def quit
    @emails.each do |e|
      if e.deleted?
        Email.delete(e.id)
      end
    end
  end
  def apop(username, hash)
    user(username)
    return false unless @user
    if Digest::MD5.new.update("#{@apop_challenge}#{@user.password}").hexdigest == hash
      return true
    end
    false
  end
  
  def process_line(line)
    line.chomp!
    case @state 
    when 'auth'
      case line
      when /^QUIT$/
        return false, "+OK dewey POP3 server signing off\r\n"
      when /^USER (.+)$/
        user($1)
        if @user
          return true, "+OK #{@user.username} is most welcome here\r\n"
        else
          @failed += 1
          if @failed > 2
            return false, "-ERR you're out!\r\n"
          end
          return true, "-ERR sorry, no mailbox for #{$1} here\r\n"
        end
      when /^PASS (.+)$/
        if pass($1)
          @state = 'trans'
          emails
          msgs, bytes = stat
          return true, "+OK #{@user.username}'s maildrop has #{msgs} messages (#{bytes} octets)\r\n"
        else
          @failed += 1
          if @failed > 2
            return false, "-ERR you're out!\r\n"
          end
          return true, "-ERR no dope.\r\n"
        end
      when /^APOP ([^\s]+) (.+)$/
        if apop($1,$2)
          @state = 'trans'
          emails
          return true, "+OK #{@user.username} is most welcome here\r\n"
        else
          @failed += 1
          if @failed > 2
            return false, "-ERR you're out!\r\n"
          end
          return true, "-ERR sorry, no mailbox for #{$1} here\r\n"
        end
      end
    when 'trans'
      case line
      when /^NOOP$/
        return true, "+OK\r\n"
      when /^STAT$/
        msgs, bytes = stat
        return true, "+OK #{msgs} #{bytes}\r\n"
      when /^LIST$/
        msgs, bytes = stat
        msg = "+OK #{msgs} messages (#{bytes} octets)\r\n"
        list.each do |num, bytes|
          msg += "#{num} #{bytes}\r\n"
        end
        msg += ".\r\n"
        return true, msg
      when /^LIST (\d+)$/
        msgs, bytes = stat
        num, bytes = list($1)
        if num
          return true, "+OK #{num} #{bytes}\r\n"
        else
          return true, "-ERR no such message, only #{msgs} messages in maildrop\r\n"
        end
      when /^RETR (\d+)$/
        msg = retr($1)
        if msg
          msg = "+OK #{msg.length} octets\r\n" + msg
          msg += "\r\n.\r\n"
        else
          msg = "-ERR no such message\r\n"
        end
        return true, msg
      when /^DELE (\d+)$/
        if dele($1)
          return true, "+OK message #{$1} deleted\r\n"
        else
          return true, "-ERR message #{$1} already deleted\r\n"
        end
      when /^RSET$/
        rset
        msgs, bytes = stat
        return true, "+OK maildrop has #{msgs} messages (#{bytes} octets)\r\n"
      when /^QUIT$/
        @state = 'update'
        quit
        msgs, bytes = stat
        if msgs > 0
          return true, "+OK dewey POP3 server signing off (#{msgs} messages left)\r\n"
        else
          return true, "+OK dewey POP3 server signing off (maildrop empty)\r\n"
        end
      when /^TOP (\d+) (\d+)$/
        lines = $2
        msg = retr($1)
        unless msg
          return true, "-ERR no such message\r\n"
        end
        cnt = nil
        final = ""
        msg.split(/\n/).each do |l|
          final += l+"\n"
          if cnt
            cnt += 1
            break if cnt > lines
          end
          if l !~ /\w/
            cnt = 0
          end
        end
        return true, "+OK\r\n"+final+".\r\n"
      when /^UIDL$/
        msgid = 0
        msg = ''
        @email.each do |e|
          msgid += 1
          next if e.deleted?
          msg += "#{msgid} #{Digest::MD5.new.update(msg).hexdigest}\r\n"
        end
        return true, "+OK\r\n#{msg}.\r\n";
      end        
    when 'update'
      case line
      when /^QUIT$/
        return true, "+OK dewey POP3 server signing off\r\n"
      end
    end
    return true, "-ERR unknown command\r\n"
  end
end

a = POP3Server.new(2226,'',4,$stderr,true,true)
a.hostname = "localhost"
a.start
a.join