The username portion of the recipient's address is ordinarily converted to lowercase before being tucked into $u. The $u is usually used with the A= delivery agent equate of a delivery agent:

Mprog,  P=/bin/sh,   F=lsDFMeu P,  S=10, R=20, A=sh -c $u
                              
                            note

Some delivery agents, such as the prog agent, execute programs. They require that the program (user) name be left as is (otherwise, the program name would not be found). Specifying the F=u delivery agent flag tells sendmail to not convert that name to lowercase.^[35]

^[35] This flag can be useful at sites that assign case-sensitive usernames (that is, eric and Eric are names of different users).

Beginning with V8.7 sendmail, the F=u delivery agent flag also determines how some aliases are treated. If it is set, usernames are stored in the aliases database without conversion to lowercase. If it is clear, they are converted to lowercase.

Also, if the F=u delivery agent flag is set, looking up the owner part that follows the owner- in a mailing list is done in a case-sensitive manner. If the F=u delivery agent flag is clear, the owner is converted to lowercase before being looked up.

In general, the F=u delivery agent flag should be set in all delivery agent declarations, except possibly the local delivery agent.