The F=M delivery agent
flag is used by sendmail.cf header commands to
force the inclusion of message identification information (F=):
The F=M delivery agent flag has no special
internal meaning to sendmail. It is a convention
that is used only in the assorted Message-ID:
header definitions (Message-ID:). See F= for a general description of this process.
Note that the Message-ID: header definition should
always be included in the sendmail.cf file
because many software packages expect the presence of that header.