Resolved. For others, this is the simple solution which works for me:
/etc/dovecot.conf: auth default {
userdb passwd-file {
args = /etc/passwd
}
passdb passwd-file {
args = scheme="MD5-CRYPT" /etc/dovecot.passwd
}}
/etc/dovecot.passwd: bob@vhost.com:$1$yAOjs09l$...::::::userdb_user=vabc_bob (the encrypted password "$1$yAOjs09l$..." comes from /etc/shadow)
/etc/passwd: vabc_bob:x:20838:2362:bob:/home/mailusers/vabc_bob:/sbin/nologin
"abc" could also be the domain ("vhost"), but this might be too large for really long domain names. So we rather use a short unique prefix for each domain.
Cheers Henry
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