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.