[Arainach]

Integration, management, accessibility, auditing and compliance for starters.

Tech folks love to say "X is is perfect substitute for Y" in the same way they say "what's so hard about X, I could write that in a week" by considering only simple ideal workflows.

The real value, and the real moat for these products, is that they're tied in with SSO and other products, easy for organizations small to huge to manage, have dozens or hundreds of specialized features crucial to large organizations. They comply with legal regulations, accessibility standards (which ARE legal regulations in many jurisdictions such as if you want to sell to the government), and more.

Writing an email server is perhaps 2% of the work involved in building an email product.