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