Depends on your auth method. Check the auth default section of dovecot.conf.
I use a custom passwd-file which has both user and passwd information contained (quite separate from the system's passwd files). The format is documented on the Dovecot site and/or wiki. I don't use PLAIN but as an example the file (which is generated from our custom control panel) has the following format:
If you are using ldap or pam or some other method you'll need to consult the docs on that.
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