Note that although /var/spool/mail is currently the standard place to put the mailbox files, some mail software may have different paths compiled in, for example /usr/spool/mail. If delivery to users on your machine fails consistently, you should try if it helps to make this a symbolic link to /var/spool/mail.
There are two addresses smail requires to exist: MAILER-DAEMON and Postmaster. When generating a bounce message for an undeliverable mail, a carbon copy is sent to the postmaster account for examination (in case this might be due to a configuration problem). The MAILER-DAEMON is used as the sender's address on the bounce message.
If these addresses do not name valid accounts on your system, smail implicitly maps MAILER-DAEMON to postmaster, and postmaster to root, respectively. You should usually override this by aliasing the postmaster account to whoever is responsible for maintaining the mail software.
Andrew Anderson
Thu Mar 7 23:22:06 EST 1996