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.