INSTRUCTIONS for the redirection version. (I don't know if you can set it default for the whole domain, but redirection works).
1. Log in to cpanel
2. Click Subdomain
3. Enter a new subdomain (eg. mail)
4. Click Go Back
5. Click Setup Redirection
6. For Horde enter: hxxp: yourdomainname. here:2095/horde/login. php
For Squirelmail enter: hxxp: yourdomainname. here:2095/3rdparty/squirrelmail/src/login. php
7. Click Go Back
8. Wait a day or two to see if it worked

(it should be up sooner than that, but don't panic if it does take that long)
edit: oops I forgot to mention that you now access your email by going to mail. yourdomainname. here