Email on Acid is an awesome tool. I love their email tests, spam tests and what's better.....if you're running into an issue they will help you in resolving the issue whether it be an html code error, css, etc. which is such an amazing experience since you'll receive the standard, "sorry, that's a custom html issue and we can't help you" response from Oracle Support.
Email on Acid and Litmus will give you a good idea. Then you can check your e-mail server to insure it isn't blacklisted. You can find the IP address in the e-mail header. To get to this information it varies from client to client. Once you've found the IP you can go to the following: