imap authentication - shadow vs mysql

Hi,

I`m looking for some opinions about authentication process for the systems with more than 1000 E-mail accounts. Maybe some one could advise me what is the best authentication method.

At the moment Im storing all passwords in my /etc/passwd. My IMAP server is growing and Im planning to build new host, my question is: what is the best authentication model for IMAP server ? Do I need to store all user/passwords in mysql or maybe in LDAP ?

I`m searching for some solution which will help me to scale my system in the future.

Maybe some one could advice me ?

Greetings

Konrad