The Hash Convention For Mail System Status Codes (HCMSSC)
D. J. Bernstein,

1. Introduction

   RFC 1893 defines codes for mail delivery failures. For example,
   code 5.1.1 means that the specified mailbox does not exist.

   The qmail package sprays these codes all over the place, by adding a
   code to the text of every error message, preceded by a hash mark and
   surrounded by parentheses. It avoids using hash marks elsewhere.

2. Examples

   Here is a typical HCMSSC SMTP error message:

      421 load average too high, please come back later (#4.3.2)

   Here is part of a typical HCMSSC bounce message:

      This is looping; it already has my Delivered-To line. (#5.7.1)

   But qmail doesn't use HCMSSC when it repeats another MTA's error

      <>: does not like recipient.
      Remote host said: 550 <foo>... User unknown (#5.1.1)

3. Security considerations

   Don't take drastic action upon seeing "(#"; it might not be HCMSSC.