mail message charging? + nanog digest

After the latest round of spam in my mail box from throwaway accounts
a spate of damn junk faxes.....

Anyone implemented per message limits on SMTP mail?

1) a billing structure that allows say 1000 outgoing mail messages
  and either charges or shuts off the account beyond that.
  I wrote a series of scripts that injected each syslog mail entry
  into a Sybase database in real time at a previous job.
2) filters on dialup accounts that prevent SMTP connections to anything
  but the local SMTP server (much as there are spoofing filters
  on most dialup routers nowadays)
3) you'd have to authenticate your SMTP connections, which if they're
from your
  local dialup pool, should be nothing more than checking your real
  time accounting database. (most ISPs aren't running real time
  accounting, but probably should be just to prevent multiple account
  A feature rich Radius server seems to be sorely lacking. This seems to
  be one piece of software that many large ISPs are writing on their own.
4) you could always allow "good" customers access through if they really
  SMTP access for some reason or another.

The basic idea would be to get rid of the damn $20 dialup accounts
most of the spam that is hitting my mailbox seems to be coming from.

Anyone see any gaping flaws in this logic?

BTW, there is a nanog digest going out twice daily.
I'm willing to add any procmail filters for anything that will increase
to noise.