3 -> 0 -> delivery agent
  if F=A, then username -> 5 -> delivery agent
3 -> 2 -> R= -> 4 -> recipient
3 -> 1 -> S= -> 4 -> sender

LHS Rules:

$*  0 or more tokens
$+  1 or more tokens
$-  1 token
$@  0 tokens (V8)
$=x Any token in class x
$~x Any token not in class x  

RHS Rules:

$i  Token i
$>x Source ruleset x
$#  Delivery agent ($@ host $: user)
$[  Canonicalize
$(  db lookup
$&f expand f at runtime only

RHS Prefixes:

$:  rewrite only once
$@  rewrite and return

Mailer Flags:

0   Turn off MX lookups
3   Extend quoted-printable to EBCDIC
5   Use S5 after local aliases
7   7-bit only
8   Suppress EightBitMode=m in MIME
:   Check for :include: files
|   Check for pipes to programs
/   Check for file addresses
@   Extended SMTP
A   User can be LHS of alias
b   Add blank line after message
c   Exclude comment from $g in headers
C   Add @domain to recipient
d   Don't enclose route addresses in <>
D   Must have Date header
e   Mark "expensive" agents
E   Change extra From to >From
f   Delivery agent adds -f to argv
F   Need From header
g   Suppress From:<>
h   Preserve uppercase in hostname
i   Do UDB sender rewrite on envelope
I   Send SMTP VERB
j   Perform UDB rewrite of header recipients
k   Don't check for loops in HELO command
l   Delivery agent is local (final)
L   SMTP line limits
m   Multiple recipients possible
M   Need Message-ID header
n   No unix-style From
o   Always run delivery agent as recipient
p   Process return path correctly (deprecated)
P   Need Return-Path
q   250 instead of 252 for SMTP VRFY
r   Delivery agent adds -r to argv
R   Use reserved TCP port
s   Strip quotes
S   Assume specified uid/gid
u   Preserve uppercase for username
U   Use UNIX-style From line
w   Check for /etc/passwd entry
x   Need Full-Name header
X   Need RFC821 hidden dot

Predefined Macros:

$_  RFC1413-validation and IP source route
$a  Origin date in RFC822 format
$b  Current date in RFC822 format
$c  Hope count
$d  Current date in ctime(3) format
$f  Sender's address (processed by rules 3, 1, 4)
$g  Sender's address relative to recipient (processed by 3, 1, S=, 4)
$h  Host part of the recipient address
$j  Official canonical name
$k  UUCP node name
$l  From format (prior to version 8.7)
$m  Domain part of hostname
$n  Error message sender (such as mailer-daemon)
$o  Token separation operators (prior to version 8.7)
$p  Process ID
$q  Default format of sender's address (prior to version 8.7)
$r  Protocol by which message was received
$s  Sender's host name
$t  Current time in seconds
$u  Recipient's username
$v  Sendmail version
$w  Short name of host
$x  Full name of sender
$y  Name of controlling tty
$z  Recipient's home directory

Notes:  It is imperative that masquerading be done in ruleset 5, not 0.  I
tried to do it in ruleset zero, and this *almost* worked.  However, it would
no longer correctly get the full name of the user from the local
/etc/passwd file.