local moduleSupport for delivering and originating messages on the local machine.
defaulthost - default hostname after the @.
  Defaults to me.
virtualdomains - virtual domains. Each line is of
  the form domain.tld<tab>user
mailbox - default user mailbox. If no .courier
  file, the message is delivered to this file or maildir, relative
  to the recipient's home directory.  local will also append
  the dash-extension (if any) to the name of the directory or file
  (trailing / should NOT be specified!). mailbox may also start
  with a |, to specify a program to pipe the message to.
As the last resort, local will attempt to deliver messages to:
A) maildrop, if maildrop is detected during compilation
  B) procmail, if procmail is detected during compilation
  C) $HOME/Mailbox
The following comments apply only when either procmail or
  maildrop are invoked under these condition. The following does
  not apply if procmail or maildrop are invoked from a .courier
  file, or from the mailbox configuration file.
procmail will be invoked with the -f flag. procmail will receive one argument: COURIER=x, where x is the dash-extension. If the local address is user-foo-bar, procmail will be run with COURIER=foo. If there is no dash extension, procmail will be run with COURIER=-
maildrop ...
rewritedel - if domain is listed in locals or
  virtualdomains, will accept delivery. If domain is in
  virtualdomains, prepends 'user-' to the local part of the
  address, then works with the local part from then on. If the
  local part is a valid userid, accepts delivery, otherwise if the
  local part is user-foo, strips off the foo, and tries again to
  check for the valid userid. If a valid userid is found, after
  stripping one or more portions, takes what was removed, "-foo",
  or maybe even "-foo-bar", prepends ".courier", and checks if the
  file exists in user's home. If not, error, mailbox not found.
  Otherwise the 'host' parameter is the userid, and the 'address'
  parameter is the extension of the courier file, or just a "-" for
  $HOME/.courier.
Rewrite function:
local - calls local.defaulthost, unless address
  contains exclamation marks.
  local.defaulthost - appends @defaulthost, if domain
  name is missing.
setuids to the user indicated in the host parameter.</\p>
attempts to open and read delivery instructions from .courier file specified by the address parameter. If can't open the file, or if it's empty, message is delivered to the default mailbox. Delivery instructions are read, a line at a time. Lines ending with \ are concatenated with the next one. A delivery instruction may specify a mailbox, a forwarding address !user@host, or a program delivery |
The output module appends a Delivered-To: userid-ext@defaulthost header. If the header already exists, the message is bounced as a mailing loop.
bin/sendmail (ho-ho) to
  lib/local/sendmail. sendmail is setgid-ed to mail,
  and is a wrapper for bin/submit. If called with the
  -bs flag, an attempt to run bin/smtpd is made.