The F=M delivery agent flag is used by sendmail.cf header commands to force the inclusion of message identification information (F=):

H?M?Re-sent-Message-ID: <$t.$i@$j>
H?M?Message-ID: <$t.$i@$j>

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.