Comment by danpalmer

This matches my experience with Zendesk. We did an integration at my previous company and they were incredibly naive with respect to email setup. We had quite strict requirements around deliverability in particular, and having the Zendesk setup play nicely with our email domain setup that was tuned for deliverability. Sadly Zendesk were unable to engage with the issue beyond "just forward emails from your gmail support inbox" which was a long way behind where we were.

For a company built on email, they're not good at it.