In reply to Mike.
All the known autoresponder/newsletter tools (Aweber, GetResponse, Mailchimp, …) let you split test your subject lines and message content.
I don’t have in-depth experience with InfusionSoft but sounds like it’s most powerful email features are sending behavioral emails and personalized messages.
Most landing page creation software also has split testing capability built in (LeadPages, Unbounce) but their tracking and reporting is quite limited.
My #1 split testing tool is Optimizely. It’s free for up to 50k visitors and works on any site.