sendmail, 4th Edition

Trusted Users

Under pre-V8 sendmail, trusted users are those who are allowed to use the -f command-line switch (-f on page 241) to override the sender address with one of their own. V8.1 sendmail eliminated this configuration command. V8.7 restored it, but as a class, and uses that class only to suppress warning headers. V8.11 and above allow only users in that class to rebuild the aliases database.

Trusted users are necessary for certain kinds of mail to flow properly. For example, the rmail(8) program of the UUCP suite of programs runs set-user-id to uucp. If rmail were not to use the -f command-line switch, all mail from UUCP would wrongly appear to come from the uucp user. To circumvent this problem, rmail runs sendmail as:

/usr/lib/sendmail -f reallyfrom

This tells sendmail to show, in both the header and envelope, the message as being from reallyfrom, rather than from uucp.

The concept of a trusted user is intended to prevent ordinary users from changing the sender address and thereby forging mail. Although that intention is laudable and good for UUCP, it can cause problems with mailing lists. Consider the following:

list: "|/usr/lib/sendmail -oi -flist-request -odi list-real"
list-real:    :include:/export/share/mail-lists/list.list

The intention here is for all mail sent to the mailing list named list to be dispatched as though it were sent from the address list-request (the -f). This causes errors to be returned to the maintainer of the list (the list-request), but replies still ...