Renaming webmail.domain.com, or disabling webmail, should free up that domain. I haven’t actually tried either, but… it would make sense. If you don’t plan to use DH’s webmail, try disabling it, then adding that domain as a subdomain. Or rename it to something off the wall that you’re not likely to bother with, since you’re not using it anyway.
Yes, I have an opinion.
Get a minimum 50% off with the “haveadreamyday” promo code, and… have a dreamy day. Original, no?