[Dovecot] Authenticate user thru PAM

my mailsystem is using postfix and virtual mailbox which stored in mysql db. SMTP AUTH is using PAM, pam-mysql to authenticate against a mysql db.

I installed dovecot 0.99-14. In the dovecot example config file it mentioned that auth_passdb can check against thru PAM but auth_userdb didn't

Does it mean that auth_userdb should use other means e.g. /etc/passwd, shadow, vpopmail, etc and in my case it is mysql. Right ?