Local delivery models

The "monolithic" model: recursively expand the complete initial
recipient list (via aliases, mailing lists, .forward files) to one
expanded recipient list (mail addresses, shell commands, files,
mailboxes).  Sort/uniq the expanded recipient list, and deliver.

The "forward as if sent by recipient" model: each level of recursion
(aliases, mailing lists, forward files) takes one entire iteration
through the mail system.  Non-recursively expand one local recipient
(via alias, mailing list, the recipient's .forward file) to a list
of expanded recipients.  Sort/uniq the list and deliver by re-injecting
messages into the mail system.  Since recipient expansion uses a
non-recursive algorithm, the mailer might loop indefinitely,
re-injecting messages into itself. These local forwarding loops
must be broken by stamping a message when it reaches the local
delivery stage (e.g., by adding a Delivered-To:  message header).

The Postfix system uses a hybrid approach. It does recursive alias
expansion, but only one initial recipient at a time.  It delivers
to expanded recipients by re-submitting the message into the mail
system, so it can keep track of the delivery status for each expanded
recipient. Because alias expansion does not look in .forward files,
it cannot prevent local forwarding loops.  The Postfix system adds
Delivered:  message headers to break local and external forwarding
loops.

Delivery status management

The "exact" model: maintain on file the delivery status of each
expanded recipient: remote recipients, shell commands and files,
including the privileges for delivery to shell commands and files.

The "safe" model: maintain on file only the delivery status of
non-sensitive destinations (local or remote addresses). Deliver to
sensitive destinations first (commands, files), but do not keep a
record of their status on file (including privileges). This means
that the mail system will occasionally deliver the same message
more than once to a file or command.
