setuid

By default, the sendmail program is executed with the user ID set to 0 (setuid to root) so that it can deliver to programs that might write in a user's home directory. When sendmail is ready to execute a mailer program, sendmail checks to see if the user ID is 0; if so, it resets the user ID and group ID to the values set by the u and g options in the configuration file. The user ID and group ID are both set to 1 (daemon). You can override these values by setting the S flag to the mailer (for mailers that are trusted, and must be called as root). However, mail processing is accounted to root rather than to the user sending the mail.