There was a moment in time where maybe it was qmail:

https://en.wikipedia.org/wiki/Qmail

Ten years after the launch of qmail 1.0, and at a time when more than a million of the Internet’s SMTP servers ran either qmail or netqmail, only four known bugs had been found in the qmail 1.0 releases, and no security issues.

More on how it was accomplished:

https://blog.acolyer.org/2018/01/17/some-thoughts-on-security-after-ten-years-of-qmail-1-0/