My own experience from running a private e-mail server the past 5-6 years is that the problem more than anything else is garbage "e-mail gateway" products, like e.g. Cyren GlobalView and Proofpoint, that gets in the way.
There's a tendency to perma-reject e-mail coming from "not seen before" domains despite the e-mail passing FCrDNS + SPF + DKIM + DMARC validation, which makes it difficult for private e-mail server users to get through to people.
There's a tendency to perma-reject e-mail coming from "not seen before" domains despite the e-mail passing FCrDNS + SPF + DKIM + DMARC validation, which makes it difficult for private e-mail server users to get through to people.