Problem with authorization of to pop3

Hello,
I am using a SUSE 9.0 Enterprise Linux server and I deal with a problem when using pop3.

When I type

telnet localhost 110

then I log in normally and the following message appears

+OK POP3 localhost v2003.83 server ready

However, when trying to type

User username

I get the following error

-ERR Unknown AUTHORIZATION state command

The problem is that I am POSITIVE about the username and password. Also, in /etc/hosts.allow file, I have put

ipop3d : ALL : ALLOW

Does someone know what all this is about?