This is why you don't run software that fossilized back in the nineties. Qmail was solid code but hasn't received the support and on-going development it needed. Use postfix instead for a secure MTA.
Both are examples of extremely well designed software written with care by security minded persons.
But that's kind of the point. 20 years worth of feature accumulation is 20 years worth of additional attack surface, which is a liability if all you need is what qmail does.
Unless you count sandboxing the entire program, and allowing errors inside the sandbox as long as they don't escape. Then C-to-WASM would be a safe C compiler, I guess.
I do not believe that this is necessarily the case.
In either case - unsafe language and safe implementation, safe language and unsafe implementation - there is the possibility for bugs, and definitely security ones. Just because you can't cause integer overflow doesn't mean you can't exfiltrate petabytes of data.
I think the major security consideration should be a comprehensive review of the entire system. Regardless of the language or implementation, a regular architectural and non-functional review of all the components in a systematic way is going to provide more security gains than any other single thing. Maybe we should be focusing less on the tools we use and more on improving that workflow.