Make sure enable staged users is enabled:
Also, does your test email account have an account on your Discourse instance? It’s worth noting that an existing user has the same email-in rights as they have posting rights. Staged users can email in to any mail-enabled category, but email addresses associated with a Discourse user can’t email in to any category which the user doesn’t have access to.
Look into group inboxes if you need to blend the two and want anyone to be able to email in but not necessarily see the topics others have created.