Most likely the mail is flagged as junk because some other user on your server sent junk mail and was blacklisted. Changing how you send mail will not effect your blacklisted status because all of your mail comes from X10's server.
If you want to make sure your mail isn't blacklisted, you could try having your mail hosted elsewhere, like Google Apps (free). I will escalate this issue so that staff members can evaluate why a paid server is being blacklisted and possibly correct the situation.