1) Make sure the envelope-sender is being set to an address where you can receive any bounces. Search the forum for "envelope-sender" for more information on how to do this.
2) How long are you waiting? Sometimes it may take mail a little longer to reach hotmail.
3) Do the hotmail users in question have junk mail filtering enabled? Have they checked their junk mail folder?
I'm not quite sure what you mean here - do you mean that you don't get the registration email to this new address?