Top-level procedure:
--------------------
- Lookup SENDER
- Don't respond if:
  - SENDER is not set
  - SENDER is <>
  - SENDER is <#@[]>
  - SENDER starts with "mailer-daemon"
  - SENDER contains no "@"
- Check SENDER rate
  - Don't respond if:
    - SENDER exceded the rate limit (defaults to 1 message every hour)
- Read in header
- Don't respond if:
  - header "List-ID:" (as per RFC 2919)
  - header "Mailing-List:"
  - header "X-Mailing-List:"
  - header "X-ML-Name:"
  - header "List-(Help|Unsubscribe|Subscribe|Post|Owner|Archive):"
    (as per RFC 2369)
  - header "Precedence:" value junk, bulk, or list
  - my own "Delivered-To:" header
- Start inject process to sender
- Copy autoresponse message to sender

Check SENDER rate (directory method)
------------------------------------
- For each entry in directory listing:
  - split filename into TIMESTAMP, PID, ADDRESS parts
  - ignore filenames that don't match these parts
  - if (NOW-TIMESTAMP) > INTERVAL: remove file and ignore
  - if ADDRESS == SENDER:
    - add one to Rate
    - if Rate is above maximum, fail without creating another file
- Create a new file in the directory, named as above, optionally using a
  link instead of a new file to preserve inodes.
- Return RATE+1 (add one for the current message)