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 reconnect the account.
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.