Email is rapidly losing its usefulness as a business communication
method. Many businesses I work with have spam filters, and the spam
filters reject ordinary email very aggressively.

This is really very regrettable. As a result, people routinely fail to
to get email that I have sent to them.

In my opinion, this type of spam blocking is unprofessional. The
messages I'm sending are not spam, yet they are blocked. So we know
the spam filter is not working perfectly. Why even advertise an email
address if you're going to configure your email servers to discard
valid business email from people you've contacted in the past?