Hi there,
You can send sita/tybe-b messages with 'Avinetmail', a product by Arinc, I've used it and found it good, not sure about aftn, I think so but you'd have to check. I know that if you subscribe to jeppesen's 'jetplan' program, you can definately send both aftn and sita type messages with that. Both these programs are web-based so you can send from anywhere that there is an internet connection and that's always handy especially for pilots downroute. You don't need any additional software downloaded on you pc/laptop.
Hope this helps.
DFO.