Qmail (#numbers) : What is it?

[Scott D. Yelich]

When I get an error and get something like (#4.2.1) or
(#5.7.1) ... what do those #s mean?  I thought I knew,
but apparently I was mistaken.  Please do RTFM or FAQ
me, but tell me which FM or FAQ to read?  I can solve
the problems -- but I'd like to know what the (#s)
mean.

Thank you.

Scott