The mail system automatically handles this situation if you are set up properly.
You should have one of the following set up as a backup mail server for your domain:
a) your ISP
b) another machine at some other site.
c) another machine at your site
In the DNS, your domain has one or more MX (mail exchange) records. These records tell the world where to send mail for your domain. Each MX record has a priority number attached, with the lowest number being the preferred place to send mail.
When I send you an email, my server looks up the MX records for your domain in DNS. It picks the mail server with the lowest number, and tries to deliver the message to that server. If that server is unreachable, it tries the server with the next lowest number, until it delivers the message or runs out of servers. If it runs out of servers it holds the message and tries again later (and again and again for about 3 days).
If my server found your server, your server notes that your address is a local delivery and gives you the message. Process complete.
If my server was forced to one of your backup servers, that server notes that the message is NOT a local delivery, but IS a domain that it can accept mail for. It queues the message, and periodically tries to pass it on to a server for your domain with a lower number than he has. Eventually either it reaches your server and delivers the message, or fails for some number of days, and bounces the message back to the originator.
So the answer to your question is, if you arrange for one or more backup mail servers, you do not need to worry about being offline for several hours, because you get all of your mail when you come back up.
Good luck.
