I did find where the mail is going. I found it in /Maildir/new/
Yes, that's how Maildir mail works - delivery to a Maildir folder means that the mail is put in Maildir/new until it is seen, when it is moved to Maildir/cur via Maildir/tmp - it's complicated, but it's necessary in order to maintain appropriate locks on the files when multiple clients are accessing them.
Still can't login to roundcube.
When you say "can't login" - what is the error?
What do the dovecot logs say? You can see where dovecot logs to by looking in /etc/dovecot/conf.d/10-logging.conf - can you see your roundcube install attempting to authenticate?
P.