If you need custom domain email hosting, don’t pay a dime for email hosting at your domain registrar or web hosting company. Why?

Because Google has probably the best email hosting infrastructure in the world and they give it away for free – up to 50 users. I think it’s a best in class product.

How good is their infrastructure? They just recently changed their service level agreement (SLA) to zero minutes of planned down time / maintenance windows for paid clients. For last year 2010, Google claims 99.984% up time meaning that their system might have been unavailable for 7 minutes each month which is negligible.

Google mail hosting aka Google Apps is free and has almost the same exact interface for the Gmail interface that we’ve all come to love with priority inbox, labels, threaded messages, integrated calendar, and keyboard short cuts. If you are transitioning from regular Gmail, make sure you turn on labs feature (like the ability to embed images and chat user list on the right sidebar) by going to Settings -> Labs.

So how do you get started? Go to the Google apps (free edition) site, click “Get Started” to the right. They even have docs on how to convert your server from MS Exchange or Lotus Notes. Fill out your domain name and follow their nice start up guide.

The most difficult thing will be modifying your DNS information to setup MX records, but they have comprehensive step by step instructions for many web hosting providers.

Another advantage of Google apps is that there’s an app marketplace which lets you extend your Gmail with CRM, project management, and other add ons that are not normally available in the regular Gmail system. The only drawback that I can see for now is that unlike consumer Gmail you start with 7 gigs and you can’t buy more mailbox storage unless you upgrade to their $50 per year per user plan versus consumer Gmail which is only $5 for 20 gigs.

And my final favorite feature of Google hosted email is the ability to make free telephone calls within the Gmail interface to anywhere in the U.S. Just use your built in microphone and speaker or headset to make quick calls to your contacts from your contact address book.

Remember though 7 gigs is still a huge amount of space. Other than that I can’t think of any other reason not to use Google apps!