On 26.8.2012, at 18.40, Alex wrote:

is it possible to configure dovecot so that it only serves mailboxes of specified users? All other users should be completely ignored so that no indexing of messages or other resource consuming actions take place.

You can configure passdb/userdb to give different settings to different users. So maybe for some users you return mail_location with :INDEX=MEMORY and don't accept their passwords. Depends on your setup what exactly you need to do..