I created some new users in my MySQL database using postfoxadmin. The passwords looked correct in the database and show the right prefix {SHA256-CRYPT} but logins were failing, so I manually created new passwords from the command line:

doveadm pw -s SHA256-CRYPT -p “a password" -u kremels@kreme.com {SHA256-CRYPT}$5$ZKsovTv0Ddc9k4…

I then went into the postfix database and manually updated the password field in the database.

update mailbox set password = '$5$ZKsovTv0Ddc9k4…' where username like 'kremels@kreme.com';

SOME of the accounts can now login, but not all.

I get a debug line like:

dovecot: auth-worker(26599): Debug: sql(kremels@kreme.com,xx.xx.xx.xx,<rnadomlookingstuff>): SHA256-CRYPT(a password) != '$5$ZKsovTv0Ddc9k4…

-- 'They've given us the answers,' he [Carrot] said. 'Perhaps we can find out what the questions should have been.' --Feet of Clay