Sometimes inboxes may get disconnected from the QuickMail account and no emails can be sent out from the account.
You may get a notification from QuickMail about the inbox losing its permission or the access token getting revoked.
This also means replies and bounces will not be detected until it's fixed.
This may be caused by a password change in the email account, cancellation of the account, or a security check by the email provider (which may happen to newly set up email accounts).
How to re-authenticate inboxes:
Go to Settings -> Inboxes -> Click the red inbox -> then click "Login with Microsoft/Google"