Under certain circumstances, before sendmail delivers to a file or through programs, it can assume the identity (user-id and group-id) of the controlling user (see Section 12.2.2 for a description of this process). Beginning with V8.7, the F=o delivery agent flag changes this behavior. Specifying the F=o delivery agent flag causes sendmail to assume the identity of the recipient. Omitting the F=o delivery agent flag causes sendmail to assume the identity of the controlling user where appropriate. (See also the F=S flag (F=S) as a way to have sendmail assume specified user-id and group-id identities.)

In V5 and earlier configuration files (Section 17.5), this delivery agent flag is automatically set for the prog and *file* delivery agents. Note that the U= delivery agent equate (U=), when specified, always overrides the controlling user.