The email-to-SMS gateways are generally reliable, given a few conditions (read below). Whether you should use an email-to-SMS gateway or an actual third-party provider depends on the application. If you're going to need a lot of control over the formatting of the messages, collecting replies, etc., then go with a third-party provider. If it's a simple notification you are sending your users, go with email-to-SMS.
If you send too many messages from the same email address, the messages will start backing up. For instance, I sent a message out to about 3,000 AT&T Wireless recipients, and some received the messages up to 3 days later. But typically, sending 10-30/minute won't cause any problems.
I use email-to-SMS gateways for an iPhone application that I built. It sends probably 10 messages per minute to almost all U.S. carriers, and it rarely experiences problems.
There are three main issues that I run into with the email-to-SMS gateway:
- You don't have control over what the message looks like. Every carrier is different. For some, the recipient will show the email address and others will show the name. Some include the subject line of the email, some don't.
- You can't control what number the messages come from. Fortunately, the general population typically reads all text messages sent to them regardless of the sender's number.
- You can't control the replies without a server-side script to manage incoming email. For most carriers, a reply from the phone goes back to the sender's email address.
