But that’s only an MTA i hear you cry, Exchange does both MTA & MDA! Bear with me.
Postfix is software to learn from. It might be written in C but the architecture is the epitome of beautiful modular design. It’s not just the meticulous separation of concerns, the care and attention to detail, everything from string handling to memory management is pristinely handled. https://github.com/vdukhovni/postfix
Even at runtime the beauty of the architecture allows for a sysadmin to choose (via master.cf) exactly how the components should be composed to fit their needs. The defaults are crafted for minimum fuss if you just need to get it running ASAP. The software is ergonomic in addition to being artfully crafted.
So what does all this care and attention get you? Only 9 CVEs in 22 years, only 3 of which are code exec, only 2 of which are (maybe) remote code exec, only 1 of which is unauth user RCE - but very hard in practice to exploit.
Maybe it’s just not that popular? It was 1/3 of all SMTP servers on the internet according to a 2019 scan.
So it’s the best MTA ever to exist, but what about MDA? Well, that was the whole point. Compose well crafted components together to build a system. You especially don’t run part of your mailserver’s web interface in kernel space because, well i’m not sure why IIS/Exchange does that :-)
So Postfix does about 1/10th of what Exchange does, and is secure. Very well, do one thing and do it well.
You talk about composing it with other stuff to create a system, but fail to mention if that system will still be more secure than Exchange. Even if each component of the system is individually very secure, that still doesn't tell you much about the security of the system. It's extremely easy to piece together two secure components and obtain 0 security.
Edit: accidentally said 'not secure' instead of 'secure' in first statement, completely changing the meaning. Corrected in-place.
Except this bug is an ssrf in the exchange web interface, so the MTA is equivalently safe to postfix. You could compose exchanges MTA with another MDA and get exactly the same security posture. Except with exchange, which is actually a good MTA.
