I intend to develop an SMS gateway in c#, but i am doubtful about it's feasibility, because my initial research had shown that an SMS gateway had to cover for protocol differences.
So what exactly a gateway had to do, further if i use SMPP, so is it possible to send/receive SMS to/from any number in the world by simply using SMPP ?
A "true" SMS gateway will need to generate an SMPP request in the the correct format (yes there are several types like CIMD & EMI) and dispatch it to the mobile networks SMSMC for sending.
You would need a direct connection to, and contract with the network(s) you use on the back end. The countries you would be able to send to will depend on the what the network provider supports/allows.
Creating one is difficult & expensive, why not use an SMS aggregator who have already done all the hard work, for example http://www.clickatell.com/.