That is simply an issue with your networks.

We have 5 Major providers here and about half a dozen or so Virtual Providers. Sending MMS between networks was only ever an issue in its very early infancy and is a widely used feature (at leats in the uk) whether you use it or not its a handy and quick way to receive small pics particually ones taken on the phone when outside the 10m range for bluetooth.

Nokia haven't provided an e-mail Daemon on the phone so if you close the app by accident or forget to load it you dont get notifications (nice one Nokia). The E-Mail client also doesent have any options such as auto send and receive every 10 mins and other features such as prompting to resizing images down so your not sending a 5mp photo and wasting data when a VGA resize would surfice. As a replacement I cant even call the built in client a workable replacement.

Also most phones dont have an e-mail address assigned to them (or not one that's easy to "just use") where as their mobile number is always a good start.

fMMS is a nice concept however development on it seems to have dried up. Their are still a lot of issues to resolve such and problems with it. When I send a message 20 times out of 30 it says "FAILED" and sends anyway and receiving messages is always based on fluke. If these issues were resolved it would be a workable solution.