Can you actually list these constraints? If not all, at least some examples? I've not encountered any I'm not handling [1]. My code has been running on various device types and under all sorts of loads for years, and have yet to encounter something not handled as described above.

[1] If you're referring to guaranteed delivery in the sense of guaranteeing that your message is delivered to a person that is guaranteed to action the message, then this is not the same problem that message queues attempt to solve. Such a scenario requires non-delivery timeouts and re-routing along a routing table, culminating in what the military world calls a "guaranteed action point", which is staffed by real people 24 hours a day.

I built a prototype based on Exchange and SharePoint once that tries to do this. It's difficult, but not impossible. The real issue we faced was cultural, in that a commander sending a message just "felt better" when sending a courier on a motorcycle, as opposed to clicking send in Outlook.