Linux RedHat Fedora Core.
Installed Dovecot without problem

Configured login as plaintext

When I try a login with
telnet localhost 143
1 login user_name password
1 NO Authentication failed

the password file contain only one user:
usernameassword

Enabled any debug, password file is readed (I see the message on maillog file)

password is in plain text

I have found a very strange record on maillog I can't understand:
Dec 21 17:40:54 srv1 dovecot: auth(default): pam(username,::ffff:127.0.0.1): pam_authenticate() failed: Authentication failure
Dec 21 17:40:54 srv1 dovecot: auth(default): passwd-file(username,::ffff:127.0.0.1): Password mismatch
Dec 21 17:40:54 srv1 dovecot: auth(default): passwd-file(roberto,::ffff:127.0.0.1): CRYPT(123456) != '123456'
Dec 21 17:40:56 srv1 dovecot: auth(default): client out: FAIL 1 user=username



Any idea on how I fix the login issue ?

Please post your dovecot.conf

The information that might help you to fix problem is available here