The Trusted Users Section:

Trusted users are allowed to change the sender address when they are sending mail. Trusted users must be valid usernames from the /etc/passwd file. The trusted users defined in the sendmail.cf file that comes with your Linux system are root, uucp, and daemon:

Troot
Tdaemon
Tuucp


The T commands define trusted users. The list of trusted users is stored in class t. Thus the three previously listed T commands could be replaced by three C commands:

Ctroot
Ctdaemon
Ctuucp


Likewise, an F command can be used to load class t from a file. The "Trusted users" section of the sendmail.cf file on our sample Red Hat Linux 7.2 system does exactly that with the following command:
Ft/etc/mail/trusted-users

A quick check of /etc/mail/trusted-users shows that the file is empty. Therefore, the Red Hat system uses only the three trusted users (root, daemon and uucp) used on most system. To add trusted users to our sample system, place the usernames in the trusted-users file.

Do not modify the Trusted Users list without a very good reason. Adding users to this list is a potential security problem.