We have noticed an issue with new Yahoo! signups (and Yahoo users who are trying to login to their SaneBox accounts) where Yahoo! will block our connection to your account.
To resolve:
1. Login to your Yahoo! Mail account in a separate tab
2. Hover over your Name and select "Account Info"
3. Select "Account Security"
4. Click on the Toggle next to "Allow apps that use less secure sign in" to turn it on.
5. Try to Sign up Again
We are currently working to correct this so that this does not affect our Yahoo! users in the future. We should have this released very soon.
If you are still having trouble signing up, please contact our support team here: support@sanebox.com.
* Note: We have noticed this is happening with users already signed up for SaneBox too. It appears to be a new feature that Yahoo! has put in place.