qmail in 32-bit mode (which was the default when it was written) had no issues. One issue was found in 64-bit mode without memory limits (against the explicit recommendation).
Most "safe" languages like Python are written in C and are full of segfaults, mostly due to the high churn rate and the attitude of the developers.
I haven't tried Rust yet, so I won't comment on that.
I find it quite brave to trust the OpenBSD guys by default. Historically, they have way too many forked huge projects (Apache, gcc, patched clang, ...) to understand them in depth.
OpenSMPTD had its fair share of exploits. sudo had its fair share of exploits.
Most "safe" languages like Python are written in C and are full of segfaults, mostly due to the high churn rate and the attitude of the developers.
I haven't tried Rust yet, so I won't comment on that.