Yeah, those are similar but narrower scope tools. They do one thing, sit behind a firewall, listen on port 25 and forward SMTP submissions to sendgrid/mailgun's endpoint using an API key.

Posthorn adds an HTTP form endpoint (which is safe to expose to the internet), a JSON API with Bearer auth and idempotency, and a provider abstraction so you can use a broad range of providers via config.

If you just need to forward SMTP requests behind a firewall to a single provider, those are far simpler solutions and Posthorn is overkill. The moment you need anything outside of that, I couldn't find a solution that worked and that's the gap I built Posthorn to fill.

Happy to take any feedback on my approach, either here or in GitHub issues.