If the mail spool file (be it /var/mail/$USER or a pathname given with the dir= parameter) is a directory then pam_mail assumes it is in the Maildir format.
The session and auth (on establishment and deletion of credentials) module types are provided.
Add the following line to /etc/pam.d/login to indicate that the user has new mail when they login to the system.
session optional pam_mail.so standard
pam.conf(5), pam.d(5), pam(7)
pam_mail was written by Andrew G. Morgan <email@example.com>.