[Dovecot] best choice of user database file to work with postfix?

[Phil Howard]

I'm setting up a Postfix and Dovecot combination.  What I want to do is have
a user database that (1) is not running from some engine (so not LDAP or SQL
or such) ... and (2) is completely disassociated from system users (e.g.
most email users are not in /etc/passwd and most /etc/passwd users are not
email users).  Ideal would be a one-file solution, which can be managed by
text editing or simple command line tools.  But what I want is ONE file that
both Postfix (for valid recipients) and Dovecot (for user login
authentication) can use together.  An alternative is some way to get Postfix
to go through Dovecot to query for users (at the time of mail arriving on
SMTP so it doesn't queue anything that would later be rejected).  This is a
smallish setup on one server, with probably a max of 50 to 100 users and 50
or so role account mailboxes over the next year or two.  Any
recommendations?