Hi Ali,
Unfortunately in most cases there is no way to guarantee that you will not lose any email if your website is down because your website might go down for different reasons (ie. expired domain name, server problems, content related problems etc.). Your web service and email service are generally handled by the same server so if it is a server related problem, it could affect both services.
We do however offer a solution that would seperate your web and email service. It is called Hosted Exchange. The Hosted Exchange service handles your email on a seperate Exchange server so it will not be on the same server as your website. This will reduce the chances of both services being unavailable at the same time.
For more information regarding Hosted Exchange please visit our website.