The username portion of the
recipient’s address is ordinarily converted to
lowercase before being tucked into
$u is usually used with the
delivery agent equate of a delivery agent:
Mprog, P=/bin/sh, F=lsDFMe
uP, S=10, R=20, A=sh -c $u ↑note
Some delivery agents, such as the
execute programs. They require that the program (user) name be left
as is (otherwise, the program name would not be found). Specifying
F=u delivery agent flag tells
sendmail to not convert
that name to lowercase.
Beginning with V8.7 sendmail, the
F=u delivery agent flag also determines how some
aliases are treated. If it is set, usernames are stored in the
aliases database without conversion to
lowercase. If it is clear, they are converted to lowercase.
Also, if the
F=u delivery agent flag is set,
looking up the owner part that follows the
in a mailing list is done in a case-sensitive manner. If the
F=u delivery agent flag is clear, the owner is
converted to lowercase before being looked up.
In general, the
F=u delivery agent flag should be
set in all delivery agent declarations, except possibly the
local delivery agent.
 This flag can be useful at
sites that assign case-sensitive usernames (that is,
Eric are names of