[Typo3] force HTML for Direct Mail plain list group

[Eric Blom]

Hello Everyone,
I've got a list of email addresses in a spread sheet that I want to  
send direct mails to. It is easy enough to  copy and past those  
addresses in to a "plain list" Direct Mail group, but, I can't figure  
out how to force all of those addresses to receive HTMl emails.

Is there a way to set the text vs HTML email preference for an entire  
Direct Mail group?

Thank you.

Eric