Let's face it, if you're a small shop, building a redundant Exchange infrastructure is way too hard and way too expensive.  So what if you have a simple Exchange setup with just a few users?  You can prey that you server will never go down and you can plan the reboots after the critical updates installs on Sunday mornings.

But why not leverage Gmail itself?  Yep, why not use Google's Gmail as the backup email server?  Sounds wacky?  It is but it works AND it's free!!!

OK, how you I do that?

First, you must have access to the domain's DNS entries because you'll have to modify them.

Point your browser to www.google.com/a, do a little reading and discover what is Google Apps.

Next, create an a free account.

Use your existing domain

Follow the steps that let Google verify that you really own the domain name.  That will involve either putting a text file in the root directory of your Website or adding a CNAME.

I use GoDaddy to register my domain names and they let you tweak the domain settings using their Total DNS Control Panel.  Check if your registrar let you do this.

After the domain ownership has been verified, you need to change the MX records (Mail Exchange).  Gmail lists instructions for a few registrars.

Basically, you need to add Gmail's MX records to the domain's MX records list.

The trick is to leave the one with PriorityValue 0 (zero) pointing to YOUR email server.

This way, mail will flow to your email server and Gmail will catch them only if your server is down.

The downside:
You'll have to create a Gmail account for each users.
You'll have to ask them to check their Google apps Gmail account when you have downtime.
The accounts are not synched.

To simplify this, you can add a CNAME to point directly to Google apps Gmail.  Something like gmail.mydomain.com
 

This is a great solution for small shops, best of all, it's free!