Dragster uses the eMail application you have set in your preferences. To change it to your preferred application, open Mail. Then choose Preferences from the Mail menu. In the General pane set the eMail reader to the application you prefer.
Unfortunately, Thunderbird is not compatible with Dragster as there is no way for another application to create a mail for Thunderbird with an attached file.
Until this changes, or we can determine a way around it, the Send to email option will not work in Dragster with Thunderbird as the default mail client.