[singlow]

They are mostly obsolete. They might be useful for low-volume, low-priority messages or hobbies, but delivery is inconsistent. Sometimes they take a while to deliver, they often get dropped or throttled.

I worked with an app that sent weather alerts to subscribers and had used these gateways. It wasn't very high volume, only sent messages during active storms in the subscriber's area, and not more than one every 10 minutes or so. We hardly ever sent more than 100 messages in an hour for all subscribers, even during a heavy storm period. Still we encouraged users to use a push messaging app like pushover which we integrated with, since sms through smtp often didn't deliver or took hours to arrive. We considered setting up a real sms service like twilio but the push messaging apps had better features than sms anyway, so the sms over smtp option was just a hack for users that really wanted sms.

In 2011 they worked pretty well, but by 2018 they were frequently down or didn't work properly. Pretty sure that messages sent through them are much more likely to hit spam filters or get rejected for whatever reason.