I am a system administrator for Jira Cloud, and I have one Service Desk project that is behaving strangely. Emails from non-company domains do not generate issues, even though the system, application, and project settings are appropriate. This only appears to be happening in one project.
Specifically: here are the errors I am seeing in the Service Desk Mail Handler logs under our problematic project.

For comparison, here are the email logs from another Service Desk project with identical Customer Permissions settings.
As you can see, emails from external sources seem to be successful in this case.
So, in order to address the "Failed: Signup is not currently available" error I first checked Global Customer Permissions here, and they appear to be configured correctly to accept issues without requiring signup.
In addition, I checked the Customer Permissions settings at the project level, and they also appear to be configured appropriately.
What am I missing? Is this a JSD Cloud bug? I have a good amount of experience in this product and yet, after consulting all the relevant documentation, I'm not sure what is going on here.