Trusted users are those who can run sendmail with the -f command-line switch to specify who sent the message, without generating a warning. Prior to V8.6 sendmail, such users had to be listed with the T command (Section 10.8.1.1). That command was ignored in V8.1 through V8.6, and with those versions anyone could use the -f switch. Beginning with V8.7 sendmail, the T command was reintroduced, but it now causes the list of trusted users to be added to the class $=t. Now, any user who uses the -f switch and who is not listed in class $=t will cause the following error message (X-Authentication-Warning:) to be included in the outgoing mail message (if the PrivacyOptions option, PrivacyOptions, has authwarnings set):

X-Authentication-Warning: user set sender to other using -f 

See the use_ct_file feature (FEATURE(use_ct_file)) for an easy way to add users to this class using the m4 technique.