Well of course, this is possible by having the email account holder go to http://www.yourdomainname.extension/webmail/ and logging in with their email account username and password.

NOTE : The username has to be the full email address, i.e. username@yourdomain.extension.

Once logged it, they will see Horde, SquirrelMail and RoundCube, as well as links to Change Password, Forwarding Options, Autoresponder Options, Configure Mail Client, eMail Delivery Route and eMail Filtering.