Prior to V8.7
sendmail, only the local
delivery agent could recognize the | character as
a directive to pipe the mail message through a program (see Section 12.2.3 and Section 13.7.4).
Beginning with V8.7, any delivery agent can be made to accept the
leading | character. If the F=|
delivery agent flag is present, the delivery agent will accept the
leading | character and call the
prog delivery agent to pipe the message through a
program. If the delivery agent flag is absent, this ability to pipe
is prohibited. In general, the F=| delivery agent
flag should be present for the local and
local-clone delivery agents but absent for all others. In
configuration files prior to Version 6 (Section 17.5), this delivery agent flag is automatically
set for the local delivery agent.
This delivery agent flag is legal only for addresses contained in
~/.forward files or
aliases(5) databases. Any
|program address in an
SMTP dialog or on the command line will be rejected.