def delete
      @command.dele @number
      @deleted = true
    end
Marks a message for deletion on the server.  Deletion does not
actually occur until the end of the session; deletion may be
cancelled for _all_ marked messages by calling POP3#reset().

This method raises a POPError if an error occurs.

=== Example

POP3.start('pop.example.com', 110,
'YourAccount, 'YourPassword') do |pop|
n = 1
pop.mails.each do |popmail|
File.open("inbox/#{n}", 'w') do |f|
f.write popmail.pop
end
popmail.delete         ####
n += 1
end
end