Many large service providers such as Verizon, AOL and MSN are implementing aggressive filtering. In many cases, the customers of these providers can place the email address you use for notification in their contact list and then your emails will be allowed through. If this doesn't work, you should contact the provider in question and ask to be placed on their whitelist. Some providers will charge a listing fee for this.

In addition, AOL (and possibly others) will not accept emails from domains that do not have reverse DNS setup. It's possible these emails are being placed into a spam folder or are being filtered.