Today, my wife just received a message I sent on July 28. I had sent a message subsequent to that one, which was delivered timely and I have had the device on for about an hour hike a couple of days ago. I received no warning when I turned device off that there were unsent messages.
Just a heads up.
UPDATE: I still have one message that shows sent on device, but not on website. Wife has not received it. That was the "subsequent message" above.