Don't use Exchange? Cool, what should we use instead? Does it support 15 people all the way up to 150000 people? I used to run Exchange cluster for 70k people, is there other mail software out there complete with non-shared disk redundancy? Where the users connect to single endpoint and software figures it out from there?
Sharepoint with another 2 RCEs. Not shocked, the software is terrible. However, it's only software that will stand up under load and let us shard it easily. All open-source software is one of those, runs fine in Homelab, likely falls down under load. Few Open Source Developers want to work on this stuff which I get because it's tedious work interfacing with computer illiterate end users. I'd rather chug sewage then do this work for free.
Finally, it's somewhat backwards compatible. Most businesses are filled with ancient software that no one has worked on in 20 years. That Excel document with Macros from 1997. With some registry changes degrading security posture, still works. I doubt you will find Office software with level of backwards compatibility unless they are using Microsoft Office level of compatibility.
Microsoft has real gordian knot here and few solutions besides "Backwards compatibility is OVER. Upgrade to modern or GTFO". Meanwhile, I get hit up by $ThreeJobsAgo over some Exchange Web Services solution I slapped together for them in Python they wanted me to upgrade to GraphAPI since Microsoft turned off Exchange Web Services in Office365.