Use email. Everyone has an email address and it's socially awkward for them if they don't respond. Also has all the benefits of being an open protocol rather than a corporate garden.
It even has E2EE, as long as people use the same client (or a compatible one).
Out of corporate contexts, email is only used to register for services, newsletters, and recover passwords. It's a shame, I prefer email over messaging for anything non-urgent.