How Fetchmail made me a spammer

[Benjamin Lutz]

Last night, I got an email from one of my users for whom I handle emails. He 
said that friend sent him a large email, which was rejected because of its 
size; and that his friend now gets a notice to that effect every minute.

What had happened?

1) The friend sent a 20MB Email to my user's public email account.
2) Fetchmail downloads that 20MB email from the public POP3 server.
3) Fetchmail tries to pass the email to the local postfix server.
4) Postfix refuses the email with a permanent 552 error because
   it's larger than 10MB.
5) Fetchmail generates and sends a rejection notice, but does not
   delete the 20MB Email from the POP3 server because the
   "softbounce" option is still the default.
6) Fetchmail sleeps 60 seconds.
7) Continue at step 2).

The damage done:
- roughly 20GB of bandwidth wasted by downloading the 20MB email over
  and over.
- an estimated 1000 rejection notices sent to the poor guy who originally
  sent the 20MB email (well, that should teach him not to send big mails! :)
- personal embarassment.

The lessons learned:
- I need better monitoring. I already monitor postfix's queue size and
  get alerts if it goes above a certain size, but in this case, the email
  in question never ended up in the queue. Monitoring bandwidth usage at
  the firewall and mails-per-hour at the mail server (which includes error
  notices) should let me detect sooner that something is amiss next time.
- Postfix's default 10MB size limit seems outdated seeing how internet
  connections have become faster; I've upped it to 50MB.
- Fetchmail's defaults are dangerous. The softbounce option, which is the
  default (the manpage claims it'll be disabled by default with the next
  version,) can generate large amounts of spam.

Cheers
Benjamin