The F=P delivery agent flag is used by sendmail.cf header commands to force the inclusion of return-path information:

H?P?Return-Path: <$g>

The F=P delivery agent flag has no special internal meaning to sendmail. It is a convention that is used only in the assorted Return-Path: header definitions (Return-Path:). See F= for a general description of this process.

The sender's envelope address (the address that would be used to return mail if it were bounced, for example) is placed into $g for use in the Return-Path: header line. This is usually done during final delivery, although it can also be done for delivery agents that lack a clear envelope address. The form of the address in the $g sendmail macro ($g) depends on the setting of the F=p delivery agent flag. Note that this is normally the same as the address in the five character "From " line.

This F=P flag should be used only with delivery agents that perform final delivery (such as local, prog, and *file*), and which do not add their own Return-Path: header. This F=P flag should not be used for any delivery agent that delivers using SMTP.