When accessing the webmail interface you should only use your own domain name eg. http://webmail.mydomainname.com. Do not attempt to use http://webmail.freeparking.co.uk/ or http://webmail.freeparking.com as your email username will not be recognised at these domains.
Furthermore you should only use your own domain name when specifying mail servers when configuring an email client, eg smtp.mydomainname.co.uk or imap.mydomainname.com. Again using any of the freeparking domains will cause the connection to fail.