I always want to have a programmable mailserver, but not just for filtering. I hope it can also do things like calling API then react according to the respond etc. It's like a eBPF program but for mailservers.

I'm not sure Sieve is a language that enables such usage. I know maybe it's a domain-specific language, but since it's a language capable of programming the server, I would rather expect more from it than "it's just yet another way to configure your mail filter".