Is it possible that this is a brand new email address at this organization? If it is, it can take a day or so to propagate out to the internet as valid, just because it's a real address, if it's new it may not be visible to the email verification services because it hasn't been updated across all services as of yet.
If you click on the email address, not the edit but the actual email address, you will be presented with options to correct it, revalidate it or just accept it:
I don't recommend just validating it, since it can cause issues with the trusted sender score but definitely recheck the address when you feel it is legitimate, which I have done here and it's come back as an issue yet. I would wait a day and try again and/or double check to make sure it is correct.
If you have valid correspondence with this address, copy/paste that valid email address into GrowthZone and try again, there could be a typo that you're not seeing (hidden character?).