I use hotmail as my primary email account-have for over 10 years. Don't create any special hoops for hotmail/gmail/yahoo/AOL users as it WILL cost you sales.
If an order is declined, why follow up? If the order is legitimate the customer will call you or use a different card/payment method.
I don't call the number supplied by a customer unless I've verified with an independent source that the number belongs to the actual card holder.