Ordinarily, sendmail tries to preserve all RFC2822 comments in sender addresses (Section 25.3.4). Beginning with V8.7, however, RFC2822-style comments can be stripped by setting this F=c delivery agent flag. (An RFC2822-style comment is one in parentheses or text outside angle brackets.) The sender address, always without a comment and stripped of angle brackets, is placed into $g ($g) and is used with the -f or -r arguments to A= (F=r). In assembling headers, the comment is ordinarily restored to $g; but if this F=c delivery agent flag is set, the comment is left out.

The main use for this delivery agent flag is to supply just the address to programs that cannot handle anything else.^[27] Another use might be to suppress disclosure of potentially proprietary information. By adding F=c to the smtp delivery agent, for example, you can cause sender headers that are defined with $g to go out without RFC2822 comments:

^[27] This was originally added because of a bug in an early version of Lotus Notes that rejected messages that included the comment.

From: George Washington (The Prez!) <CX75G@fire.wall> without F=c
From: CX75G@fire.wall                                 with F=c

Note that this does no good at all if users send out mail with disclosing headers already present, or if they give out information in signature lines.