This is a very common feature for email clients - Thunderbird and K-9 Mail support it. It should not be difficult to implement because it’s already built in the SSL library. On Android the built-in certificate store may be used.
Today I’ve configured mandatory client certificates with my Dovecot IMAP server and was going to do it with my Postfix SMTP server too. Unfortunately I have to disable this feature because Delta Chat doesn’t support it.
async-native-tls allows setting client certificate with identity method so it should be doable.
Could you provide links to the documentation you used to configure the server or examples of configuration? Maybe client TLS certificates are somehow useful for chatmail servers, but at least I want to be able to setup a testing server to check that client certificates actually work.
It’s not easy as it requires some weird manipulations with OpenSSL tool, and I’ve only done it for Dovecot, not Postfix. I’ll write a tutorial for both, but I need some time.