It would depend heavily on what language you want to use to forward requests, and if you even want to forward or want to be a gateway. There are also services that you could forward mails to containing the SMS message, that would in turn send these to the SMS device (which is what you would want to do, in the long run, though without the external party, I'd wager).
There are varisous reseller companies that already do this kind of service, so you can look them up for a quick for-pay option. Doing it in-house takes a little more info...
For some info:
However, if you want to make yourself into an SMS gateway to do the sending yourself through yourself, you're going to need some hardware, sim cards and an antenna.