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,
needs a great
deal of information about each delivery agent.
delivery program command line Mlocal, P=/bin/mail, F=rlsDFMmnP, S=10, R=20, A=mail -d $u flags sender/recipient rules
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. 
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