Hotmail hates competition
I sent numerous invites to my friend The Nameless One at the Hotmail address provided by him/her/it. All messages went down th gulch that is the Emptiness Beyond. Had to finally send the invite to the Yahoo address. We had a similar experience when sending GMail invites to Hotmail and Yahoo addresses
Lesson learned, do not send invites use competitiors services to free accounts. The service providers hate that and reserve the right to smack you on the upside.