You could try connecting directly to the E-mail server for the particular service. (Most services document this.)

If somebody can connect to the Internet, they can directly connect to a service's E-mail server to send mail to a user on that server, and likely they can use SNPP directly, unless there are firewall issues.

Anyway, you should look at those modules I referred to. If nothing else, web gateways to SMS are not standardized like search engine interfaces, so your code may break every time the service provider decides to redesign their pages.