Here are some of the common error in sending accounts.
AUTH XOAUTH2 error code: Can't create a new access token for user
This error means that your email provider is not able to share the toke with Instantly or revoke the access of the token.
This usually happens if you changed the password for your sending account and did not update the account in Instantly. The solution for this error is to delete the account and add it again.
550 5.4.5: Daily user sending quota exceeded
This error may appears in two instances.
When accounts hit their daily sending limit set your email providers
If there's too much spammy content in your campaigns and leads have been marking your emails as spam.
The solution is to wait for 24 hours before sending any more emails. This will give your account time to reset its daily sending limit or contact your email provider to increase your daily limits.
This app is blocked
If you see this error message, it usually indicates that oAuth has not been configured correctly for the workspace associated with that account.
You will need to approve Instantly from the admin account (of the GSuite workspace where your domain is connected). Once you're there could you please Select "Trusted" and click on CONFIGURE button.
You can watch the video walk-through about on how to fix "This app is blocked" error message, by clicking here.