[Dovecot] authentication methods : pam, passwd and then backup question. How to restrict the methods

[J4K]

Dear all,

    I noticed that incoming users were attempting to authenicate using
several methods like pam sql. Eventually, they logged in.
Is there a way for dovecot to only query a mysql backup for users, and
skip the other methods?

Examples from logs (via Roundcube webmail in this case, but the same
messages appear with port 993)

Jan 25 17:22:17 srv dovecot: auth-worker(default):
pam(aa at bb.co.uk,127.0.0.1): pam_authenticate() failed: Authentication
failure (password mismatch?)
Jan 25 17:22:17 srv dovecot: auth(default):
passwd(aa at bb.co.uk,127.0.0.1): unknown user
Jan 25 17:22:17 srv dovecot: imap-login: Login: user=<aa at bb.co.uk>,
method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, secured

As shown above, the user uses pam, passwd and then finally the user
account fetched from mysql.

The distribution is Debian Squeeze.

Regards, s