Prior to V8.7 sendmail, only the local delivery agent could recognize a leading / character as a directive to append the mail message to a file (see Section 12.2.2 and Section 13.7.3).

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 a leading / character and call the *file* delivery agent to append the mail message to a file. If the delivery agent flag is absent, this ability to append 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 / file address in an SMTP dialog or on the command line will be rejected.