Update 2008-10-31
Since the migration of Hummer8 and Hummer10 we have noticed a few issue with Horde, We are working with Plesk to resolve this but so far no solution has been forthcoming.
In the mean time we have made another email program available. SquirrelMail.
You can use this by typing http://mydomainname/webmail (replacing mydomainname with your actual domain name)
Also when you use if, before you send emails you must alter some settings. Here is a tutorial that covers this
Horde is still available and sometimes you can see your emails if you keep clicking.