When you are creating your email campaign, you'll see it in a browser preview. Once you send it out though, that same email will be viewed in any number of different ways.
Desktop email applications like Outlook or Apple Mail, web based email like Gmail and Hotmail and increasingly on mobile phones and devices like the iPad.
Your email could look quite different in an email client than it does in a browser, and display differently in Outlook than it does in Gmail. To further complicate things, some web based email programs actually work differently depending on the browser you access them through.
The kinds of differences you might see include padding and margin changes, spacing between elements, background images not showing up, CSS being ignored and much more.
So it is important that you check to make sure your email is readable and appealing on as many email clients as you can. That's not to say it should look identical, but it should be usable.
How can I test my emails?
The very minimum you should do is make use of the built in 'send a test email' function to view the email in your own email program.
You can access this when setting up an email campaign. After you've selected the recipient list for the campaign, you'll reach Step 4.1 - Test your campaign.
This is perfect for quickly checking your campaign in the email programs you have available. Setup a free Gmail and Yahoo! account, for example, and use those. The last 5 addresses you tested with will be remembered for next time.