If your wanting to be notified on mobiles I would suggest you look at something like
https://pushover.net, I use it for notifications.. Anything that can send out email notifications could use it by sending an email to: <USERKey>@api.pushover.net
It works quick enough that when I go check the mail with my phone in my pocket, I barely have the mailbox door all the way open before my phone chimes to let me know I am in my own mailbox.. but I am using the API directly (vs through email gateway) as my HomeAutomation software supports it natively.
Just over a day ago I get the special "Mailbox" chime on my phone shortly after midnight, open my front door and find a guy still in my mailbox.. so it's pretty damn quick.