I am in the same boat, can't get email notification features to work. I am with ATT and they use SSL and TLS authentication for inbound and outbound email. This means they do not use the old "open" " standard" email ports like port 25 for smtp mail sending.
I have been unable to to get gmail to work for the same reason. Thunderbird can access and send mail with my gmail account, but it is using secured ports with authentication.
Dlink needs to build this into the next firmware release as no one leaves their email server running on port 25 anymore for spam security reasons.