The sendmail program does not perform the actual delivery of mail.
Instead, it calls other programs (called mail delivery agents) to perform
that service. Because the mechanics of delivery can vary so widely
from delivery agent to delivery agent, sendmail needs a great
deal of information about each delivery agent.
command defines a mail delivery agent
and the information that sendmail needs.
delivery program command line Mlocal, P=/bin/mail, F=rlsDFMmnP, S=10, R=20, A=mail -d $u flags sender/recipient rules
M configuration command is composed of six parts: a symbolic name
followed by five equates,
each separated from the others by commas. Spaces
between the parts are optional. The specific syntax of the mail delivery
agent command is
symname, equate, equate, ...
M always begins the delivery agent definition,
followed by a symbolic name (the
symname) of your choosing and a
comma-separated list of delivery agent equates.
A= equates are required. The others
The comma following the symbolic name is optional. As long as there is a space following the symbolic name, sendmail parses it correctly. The comma should, however, always be included for improved clarity. 
 SunOS 4.1.2's sendmail does not properly support freezing if the comma is present.
In the following, the first example includes the comma, and the second omits it. Both are parsed by sendmail in exactly the same way:
Mlocal, P=/bin/mail, F=rlsDFMmnP, S=10, R=20, A=mail -d $u Mlocal P=/bin/mail, F=rlsDFMmnP, S=10, R=20, A=mail -d $u