If you see:
User 'admin' - failed auth, from=127.0.0.1
in your /var/log/messages file, then you may have a newer setup for pam. Â Edit your /etc/pam.d/vm-pop3d file. Â Change:#%PAM-1.0
auth    required   /lib/security/pam_pwdb.so shadow
account   required   /lib/security/pam_pwdb.so
#%PAM-1.0
auth    required   /lib/security/pam_unix.so shadow
account   required   /lib/security/pam_unix.so
A report for 64-bit systems, the vm-pop3d file should be
#%PAM-1.0
auth required /lib64/security/pam_unix.so shadow
account required /lib64/security/pam_unix.so
Note that it's recommended to upgrade to dovecot either way.