You could change the mail dns record and setup another domain as your subdomain (mail.domain.com). Then using the Site Redirection feature, have it forward
Or a better way might be to remove the mail dns records, create a virtualhost for your mail.domain.com in the httpd.conf with a redirect in it. Although removing the mail (mx) records would basically disable pop3 mail access.
I'm not sure if this is possible but you might be able to do the redirects using a virtualhost for port 80 and somehow get the same subdomain to respond to the pop3 port. Haven tried it