So I've read a lot of the forums/help/instructions, but am still having this problem. When I use the Domain-Mapping plugin from within my sub-site (multi-site), I get the DNS not resolving error. I'm not a total DNS idiot, but I've now tried this on about 6 different domains and sub-domains, thus having tried about every combination of A/C records with my site's static IP.
What happens when I go to the sites: sub1.mymulti.com - works. Sub1.com (target of Domain-Mapping) immediately is seen in the browser info bar as loading from http://www.mymulti.com (which seems appropriate, except perhaps for the www part). But instead it pulls up http://www.mymulti.com/register as if it wants to register a new user or something, and it doesn't pull up anything to do with sub1.com or sub1.mymulti.com.
My nameservers are all set and solid, and I've had a lot of success doing this kind of thing in the past, but I'm still having problems with this plugin. And as I said, I'm also using Multi-Domain plugin, and as suggested in other answers, I've deactivated both, deleted sunshine.php, and reactivated in the given order (MD then DM).
It seems like something really simple is stumping me here. Any help would be greatly appreciated! Thanks!