Emails not visible on freshly installed CentOS 6 with Virtualmin

Main symptom of a problem is that emails are visible in the Read User Mail utility in Webmin but they do not show up in either webmail (Roundcube/Squirrelmail), Usermin or any email client configured to connect to the server.

After further inspection, it appears the emails get delivered to /var/mail/usermailbox@domain.tld instead of /home/username/homes/usermailbox/Maildir.