Definitely have the hosting company fix the forged helo problem.
As far as the MIME_HTML_ONLY BODY this is because you're sending a 2part text/html message but only including html (although it seems like the score of 2.9 might be high for that - check some other headers, are they all that high?) Most personal email clients only send html or text but not both at the same time. Get a program designed for mass-emailing and it should include support for 2part text/html. I use GroupMail - they have a free version here
Download GroupMail today! but I haven't tried it.