What are the permissions on
/var/spool/mail? It should probably be owner root, group mail, mode 1777. If that is in order, is there already a
/var/spool/mail/otheruser mail file? Is it owned by otheruser (
ls -la /var/spool/mail/otheruser).
That message is telling you that either the permissions on
/var/spool/mail or on
/var/spool/mail/otheruser are wrong.
You mention you put entries in virtusertable and generic-domains, what did you put there?
You can check that mail is being delivered to that user by sending a message to that user from your original user and checking what
/var/spool/log/maillog reports about the attempted delivery.