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++==============================+
+|The SMTP Daemon Architecture  |
++==============================+
+
+Introduction
+------------
+
+The SMTP daemon is a complete mail architecture designed to provide
+an MTA and MDA for OpenBSD systems (and others !)
+
+The SMTP daemon is configurable through a simple pf-like syntax to
+select external or local delivery and the means to do so.
+
+The SMTP Daemon relies on a fully asynchronous and event based data
+transfer model. The IMSG pseudo-protocol is used throughout the daemon
+to provide communications between separate privilege-separated
+processes.
+
+The following processes are currently implemented:
+
+	* smtp:		The unpriviledged SMTP server
+	* mfa:		Mail filter agent
+	* queue:	A queue scheduler
+	* mda:		Local mail delivery agent
+	* mta:		Remote mail delivery agent
+	* lka:		Lookup agent, handles DNS, db, file and external maps
+	* control:	External interaction gateway
+
+The SMTP Server
+---------------
+
+The SMTP Server conforms RFC 5321 and also provides interesting
+extensions such as STARTTLS (RFC 2487).
+
+
+The Mail Filter Agent
+---------------------
+
+The mail filter agent has two roles. First it decides if a mail is
+allowed to be delivered by the daemon, it then performs resolution
+to determine the delivery method that applies to the mail, either
+local or remote. The second duty is to perform optionnal modifications
+on the envelope content.
+
+
+WorkFlow
+--------
+
+The base structure that traverses most of smtpd's architecture is a
+struct msg, the typical workflow is as show below from left to right:
+
++------+                                          +-----+  
+| SMTP |======+                               +===| MDA |
++------+      |     +-----+     +-------+     |   +-----+
+              |=====| MFA |=====| Queue |=====|
+                    +-----+     +-------+     |   +-----+
+                      ||                      +===| MTA |
+                    +-----+                       +-----+
+                    | LKA |
+                    +-----+
+
+
+Client -> Server workflow:
+
+1- The message is initially received from either the listening service or
+   /usr/sbin/sendmail (for which smtpctl is a link).
+
+2- As envelope is gathered, the information is handed out to MFA for
+   validation.
+
+3- MFA takes decision as to wether or not the message can be delivered
+   locally or relayed out and notifies SMTP of its decision.
+
+4- If message is not rejected, a file descriptor is requested from QUEUE
+   by SMTP which will then write the message to the file descriptor.
+
+5- Once content is written, SMTP notifies QUEUE that the content is
+   fully written and provides a list of recipients.
+
+6- QUEUE constructs batches of messages with same message id and going
+   to same destination, and registers them to the batch queue. It then
+   writes the information on-disk so that it can recovert from a crash
+   or shutdown, and notifies SMTP that message is accepted.
+
+7- SMTP notifies client that message was accepted.
+
+
+Queue workflow:
+
+1- A batch queue traversal is done. For each batch, QUEUE computes the
+   retry time based on the number of times it was tried already and
+   the time of last attempt. A batch may be in three states:
+
+	a) it is either ready for processing, in which case it is
+	   handed to MDA or MTA for delivery attempt.
+
+	b) the delay before next retry has not yet expired, the
+	   scheduler will simply ignore it.
+
+	c) the batch age (currently 4 days) has expired, the
+	   scheduler will eventually generate a mailer daemon batch
+	   and remove the original batch from the queue.
+
+
+XXX - needs to be completed/improved/updated