Business Exchange Hosting by DataGate
http://www.datagate.co.uk/exchangehosting/

NetVigour Hosted Microsoft Exchange
http://www.netvigour.com/products_and_services/hosted_microsoft_exchange.asp

Hosted Microsoft Exchange by ExchangeMyMail
http://www.exchangemymail.com

AppRiver Exchange Hosting
http://www.appriver.com/exchange/

Intermedia.NET Hosted Exchange
http://www.intermedia.net/it-professionals/hosted-exchange/hosted-exchange.asp

Microsoft Exchange 2003 Hosting by 123Together.com
http://www.123together.com/

Exchange Hosting from ASP-One
http://www.asp-one.com/

MailStreet Exchange Hosting
http://www.mailstreet.com