Does Postmatic work with DNS-based firewall services such as Cloudflare or Incapsula?
If you are using a DNS-based firewall or security service such as Cloudflare our servers may have a difficult time reaching your website when we try to pass incoming comments along. If you are using one of these services and notice that mail is going out via Postmatic, but nothing is coming back in (such as subscription confirmation emails or comment replies) this is most likely the problem.
How to fix connection issues
While each service is a little bit different the general idea is that connections between our servers and your web host need to be allowed. This is usually called whitelisting.
You'll need to whitelist app.gopostmatic.com. Your best bet is to contact the support team at your firewall service and ask them how to go about doing so. Most offer self-serve control panels that will have you up and running in just a few minutes.