1. what you can do is call your ISP and see if they will queue your mail until your server comes back up. What they basically do, is add a second MX record that points to thier mail server. If your server is offline, your ISP holds the mail. Yourisp tries every 15 minutes to locate your server.
2. If that is not feasable, you can ask a hosting company to host email for your domain. Select a hoster that allows wildcard email boxes, like everybody@mycompany.com to go to one mailbox. Add thier mail server as a second entry in your MX records. So when your server is down, all your company mail will go into that one mailbox hosted by the email hoster. PS: most website hosting company provide mail hosting as an included service. One I know of is communitech.net.