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+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.