I can somewhat vouch for MailPace. I wrote the Rust library for it and talking with them was very straightforward (getting a free key to test, testing the libra

I can somewhat vouch for MailPace. I wrote the Rust library for it and talking with them was very straightforward (getting a free key to test, testing the library's functionality, etc) and he was very responsive and knowledgeable, even installing the Rust compiler to test. That said I haven't actually used it to deliver critical mail. The delivery latency being published on their home page is another nice touch that shows they are dedicated to transparency.