That's a smart move by Mailgun, I'm sure Sendgrid will get there. That's the kind of thing people don't think about with enterprise customers.

- Compliance - Support - Security (not keeping things secure - offering custom security options. SAML, user roles, permissions, IP whitelisting, etc) - Customization

Building to support big complex customers is hard and if you get it right from a product and sales perspective, the inertia you can build is massive.