Emails not being sent all the time - "Client does not have permissions to send as this sender" is the error we see when emails are not sent.
That error message is an indication that the email being sent was blocked by the outbound email server for some reason.
This is an error seen on services such as Office365, Hotmail, GMail. etc.
The error is coming from the mail server and not generated from within HEAT.
Please check your SMTP settings at your provider, and make sure that they are not blocking the emails due to email content, recipient or the volume of emails sent over a period of time.
Either of those could cause your provider to block email and give you the error.