One of the problems with running a Small Business Server is that you are reliant on your office Internet connection which, while reliable,  is does not have 100% uptime. You can however host your email on Google (privately) by registering for Google Apps with your company's domain name N.B. you do not need to change all of the hosting details for your domain over to Google just the mail related ones i.e. your MX records and mail.yourcompany.com. Once you have set this up you will need to set up an account for each user and forward the email to another (semi-private) email address for me this works like this:
1) I have the MX records for rothmancompany.com hosted on google
2) I use DynDNS.org to host a dynamic domain for my Small Business Server in the form mycompany.dnsalias.com
3) Small business server is configured to recognize myemail@mycompany.dnsalias.com as a valid internal address
4) My default email address is still unchanged
This provides you with a 100% uptime mail server that you can also access remotely it also provides you with a great backup of all of your incomming email!