The other side of this besides the delayed receiving of messages is with monitoring you want to get the alerts even if your network is down and unable to send via email to your pager, cellphone, etc. Having an out of band method to get those alerts out on criticial alerts is paramount. I've used Nagios for many years but unfortunately have never worked with sending through Verizon. I've had decent experience using Sprint's gateways sending to my phone with minimal delay.
Our solution, crufty as it might be, was that our monitoring server has a modem on it. As long as the pots lines are up, we just have it ring the on-call cell phone. When you see the caller ID, you know its time to get to a terminal. Usually our 10digit@verizon would follow 10-15 seconds later. Tuc/TBOH