Each delivery agent that is defined in
the configuration file may or may not have an L=
(line length) equate (L=). If that equate
is missing, or if the value assigned to it is less than or equal to
zero, and if the F=L delivery agent flag (F=L) is set, the default value that is used
becomes the value of SMTPLINELIM. Otherwise, the default value is 0.
This logic is there to support old configuration files that use
F=L in place of the newer L=.
The default for SMTPLINELIM is 990 (defined in RFC821), and that
value should not be changed. Rather, if you need a different
line-length limit for a particular delivery agent, you should use the
L= equate when defining it.