system (in a multi-computer BBS) the error occurred.
For the mail server:
- keep track of the number of times each mailproc handled a message
- display counters when blocking/filtering messages in log output
- advertise 8BITMIME in the ESMTP EHLO response
- do not route mail to mailproc when the execution requirements are not met
  (e.g. ARS, DNSBL, SPAM, etc.)