Fonoster might be a bit more popular than Sendblue. We know about 1 link to it since March 2021 and only 1 link to Sendblue. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
There are several providers that offer an iMessage service, like https://loopmessage.com/ and https://sendblue.co/. You could also go through Apple directly, although the process and API are not publicly defined. Another approach and mechanism can be seen in this repo https://github.com/ZekeSnider/Jared, where you can interact with the iMessage database directly... - Source: Hacker News / about 1 year ago
There's a service that's open source for full on features if that helps, here it is: https://fonoster.com/. Source: 10 months ago
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