When you try to send an email or invoice and receive an "Authentication failed" error message, this is usually related to your Microsoft/Google account token expiring.
Why does this happen?
This error occurs when your Microsoft or Google account token has expired. A token is a temporary security code that allows Uku to send emails on your behalf, but it needs to be refreshed periodically.
How to solve the problem?
To restore the connection, follow these steps:
Go to Settings
Select Emails
Click Email addresses
Find your email account (it will be marked in pink color)
Click on that account
Select "Reconnect"
After reconnecting, email sending should work again.
Resending failed emails
If email sending failed, read here how to resend emails.