To me it looks like lmtpd can't write to our system inboxes under /var/spool/mail. Probably a permissions problem; so we reset the files in this directory to be owned by group "mail" with group RW permissions. That didn't help; we still get the User unknown complaint.
lmtpd writes in a Cyrus Mail Store. It is not compatible with standard unix mbox files in /var/spool/mail, the maildir format comes closer, but cyrus still uses cache databases within these directories.
What is in your imapd.conf? Where are your imap partitions?
If you want to access concurrently with multiple clients, your only chance is to use ONLY pop3 and imap4 and do not access the cyrus mail store directly (file-based). This won't work in a useable manner and may break up your whole store. If you already did this, reconstruct(8) should be your friend.
Pascal -- Pascal Gienger, Kanzleistrasse 14, D-78462 Konstanz Postfix+SASL-AUTH+LDAP Solaris 8: http://pgienger.de/postfix/
