Over the last 2 days we've started getting the below error message

ERROR: Failed sending email to: "David Tyacke" with subject: "Order Update #2836" SMTP Error: Data not accepted.

The message appears when we try and add comments to the customers order then change the status and then send them a notification.

If we choose not to notify the customer then the comments are successfully added and no error is shown. Im presumption this is more a e-mail problem than a database entry problem.

I have checked in our e-mail options (mailbox name, password and SMTP Host) and nothing has changed. I have spoken to our hosting provider and they also say nothing has changed!

Does anyone have any suggestions
Thanks
Tony