If you create a lot of HTML emails, you surely spend a lot of time making sure those emails look as good as they possibly can no matter what email client the end-user might have. But you can’t possibly test for them all. Thankfully there is Litmus.
Send Litmus a copy of your email design, either by uploading the HTML or sending a test email. Within a couple of minutes you’ll see screenshots of your email as it’s rendered by all the different email clients. It couldn’t be easier.
To be clear, Litmus is a professional service; it’s not for the casual user. Litmus costs a minimum of $49 per month for the basic service, and goes up to $300 per month for the premium. It sounds expensive, even at the low end, but I don’t know of any professional digital design firms that don’t use Litmus (or a similar service).