The F=U delivery agent
flag causes sendmail to prefix a five-character
"From " line to
the start of the headers if there is not already one there. Whether
one was prefixed or not, this delivery agent flag also tells
sendmail to add the words
remote from host to the end
of that line, and also requires that $g be in the
form host!....
The F=U delivery agent flag is required when a
neighbor UUCP site runs an old (or possibly SysV) version of the
rmail program. The newer BSD versions of
rmail do not require this delivery agent flag.
Note that the F=n flag (F=n), if specified, overrides this
F=U flag.
