No unused E-Mail found

"No unused E-Mail found" - What is the problem and what should be done to fix it?

There are 5 emails in the project, I checked, they are all working.

Emails can only be used once per site to register an account. Once it has been used, it needs to be replaced with a new one. When resetting campaigns, you should also replace with brand new emails.

If you used catchall emails with spintax, then you would never really need to replace it, as the spintax would create thousands of different emails from the same subdomain/catchall.

I have been using free emails in gsa ser (mailmenot.io works fine so far), I usually create at least 70 emails per project and they seem to work fine. But I have to admit that I have not compared them with catchall emails. Have you guys tried those free email from gsa ser before? Or catchall emails with private domain names work better? 

If you would like to review or try the catchall emails just let me know. the cost is very effective.