in %ROOT%/site/mail_queue all emails are being processed but users did not receive the notification.
SDM 14.1 Exchange server 2012/2014
This message can be caused by exceeding their daily sending e-mail allowance (100 delivered messages via SMTP or 500 delivered messages by their web interface), or by sending a few messages to non existent e-mail addresses.
In rare cases it could also be because you haven't physically signed into your email account.
If you account has been temporarily disabled for exceeding the sending limits it's usually automatically re-enabled 24 hours later and notification emails will be triggered.
For this type of email issues please contact your Email Server Administrator to confirm user account limitation on email server side.