[Dovecot] username and password not from /etc/passwd

[Zoong Pham]

Can dovecot use usernames and passwords not from /etc/passwd and be able to 
read mails of an user whose username and password are in /etc/passwd ?

For example:
- Joe Bloke has a local Unix account joe on Unix server mail.example.com
- Sendmail on mail.example.com accepts mails for joe at example.com and put 
them in /var/mail/joe
- can dovecot use other username/password file or database or directory 
rather than /etc/passwd to authenticate Joe and let him (once logs in 
successfully) reading mails in /var/mail/joe ?
Ideally, the usernames and passwords that dovecot uses are different with
the ones in /etc/passwd. Say, username is joe.bloke at example.com

That way, Joe doesn't need to know the username and password on the Unix
server
at all.

In summary, I want to have IMAP usernames and passwords totally different
from
the local Unix usernames and passwords. And users use only IMAP usernames
and
passwords to read their mails.

I use OpenBSD-3.4, Sendmail 8.12.9, dovecot-0.99.10p0 (pre-compiled packaged
from OpenBSD).

Thanks,
Zoong Pham