Email submission: Acrobat chooses the wrong email client

Hi - I've made a simple form using Acrobat 8 Pro (with LiveCycle) that contains an email submit button. I have set Thunderbird as my default email program and doublechecked that MS Outlook is not set as the default. However, every time I test the form, clicking on the email submit button creates a message in Outlook, not in Thunderbird. How can I get Acrobat to quit firing up Outlook and use my default email program?

Thanks!