I solved it. For anyone who wants to know I su-ed to root and used Mutt to send an email locally to the user I wanted to create a mailbox for. So, by sending an email from root@mymachine to myusername@mymachine it automatically created the mailbox /var/spool/mail/myusename.
Now I just need to figure out why I can't receive mail from other hosts...
Fun fun fun!