If you don't want to sign up for a Litmus subscription, you can use Direct Mail for OS X (http://directmailmac.com http://directmailmac.com) to run design tests

If you don't want to sign up for a Litmus subscription, you can use Direct Mail for OS X (http://directmailmac.com http://directmailmac.com) to run design tests on a pay-as-you-go basis. Also inlines your CSS by default (but can be turned off). Disclaimer: I am a developer on Direct Mail.