error in mail messages when truing to receive email

[rtbates]

I am using FreeBSD 4.9, Postfix 2.0.18 and QPopper 4.0.5 with sasl and 
openssl

I keep seeing the following error

Mar 17 07:34:28 mail qpopper[43130]: bates at BatesiBook.myplace.com 
(172.18.1.139): -ERR [SYS/TEMP] POP authentication DB not available 
(user bates): No such file or directory (2)

after a timeout it then allows me to receive email....

What does this message mean?
What file or directory is it looking for?
How do I fix it?
Thanks in advance
Rich Bates