pop3 able to login, but log is authentication failure

[Jim Worke]

I can login into cyrus-imap's pop3 server.  However, I saw the log shows as 
below:

May 13 20:03:49 thunderbolt pop(pam_unix)[4918]: authentication failure; 
logname= uid=76 euid=76 tty= ruser= rhost=  user=testuser
May 13 20:03:49 thunderbolt pop3d[4918]: login: 
thunderbolt.chaos.com[192.168.1.1] testuser plaintext

Can anyone explain why is this so?  I'm using PAM to authenticate to LDAP.