Hello,
Hmm, maybe I grasp your problem now: You use a passwd file, which uses the colon as delimiter for its fields itself, right?
thats correct.
However, I never used passwd, but it's limited to uid/pwd/homedir anyway, right? So you ought to have some fixed template to reach your mailboxes relative to the user's homedirectory, which you can put into default_mail_env and add the ":INDEX=MEMORY" there.
As I said before: The Maildir-folder is more complex.
Let's say we have a mailbox called try@test.de, the it's maildir-location would be /pop3/mailboxes/t/te/test.de/_/try/Maildir.
If it would be a subdomain of test.de, lets says try@subdomain.test.de, the maildir-location would be /pop3/mailboxes/t/te/test.de/subdomain/try/Maildir.
dovecot is in no circumstances able to see whats a subdomain and whats the domain. You can't split at the dots, because errors would occur for third-level domains like test.co.uk. So the entry of userdb must be used.
Regards Marten