The Unix-style mailbox (a single file into which many mail messages are placed) requires that each message be separated from the others by a blank line, followed by a line that begins with the five characters "From ":

and thanks again. -- bill               one message ends
                                        a blank line
From george Fri Dec 13 12:03:45 2002    next message starts

Ordinarily, sendmail adds a five-character "From " line to a message if there isn't one. The F=n delivery agent flag prevents sendmail from doing this. It is intended for use when not dealing with a Unix-style mailbox, or when dealing with a delivery agent that adds the blank line and "From " by itself.

Note that if the F=U delivery agent flag is specified (but not F=n), the five-character UUCP-style "From " header line is created, and the words remote from $g are appended to that line. The F=n delivery agent flag should always be specified for SMTP delivery agents. The five-character "From " line is not a valid RFC2822 header (because it lacks a colon) and is not permitted.

Apart from SMTP, the use of the F=n delivery agent flag is best determined on a case-by-case basis. Some delivery agents always generate a "From " line, so the F=n delivery agent flag can be used to avoid duplication. Some delivery agents generate a "From " line only if there is not already one there, so the F=n delivery agent flag is optional and perhaps best omitted. Some delivery agents never generate a "From " line, yet require one (such as the uux program); for these the F=n delivery agent flag should always be omitted.