Looks like something is going wrong while decoding the HTML email (presuming it's sending in HTML). We can handle all types of emails, including HTML emails, but sometimes decoding fails for some reason.
Unfortunately we currently have no way to debug what's going wrong exactly. We do have a beta program that hopefully improves on some of Twittermail's current limitations. Would you interested in testing the beta to see if the email comes through there? If so, please send an email to email@example.com.