Email Delivery Errors -- When there's a problem delivering an email message that you sent, Office 365 sends an email to let you know. The email you receive is a delivery status notification, also known as a bounce message. The most common type is called a non-delivery report (NDR) and they tell you that a message wasn't delivered. See Email non-delivery reports in Office 365 for more information.
If you try to send a message with a blocked extension, the attachment will be removed. You will receive an error message from Postmaster with the subject of "Undeliverable message"
This message was created automatically by mail delivery software. Your email message was not delivered as is to the intended recipients because malware was detected in one or more attachments included with it. All attachments were deleted.
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Subject: attachment test
Time received: 6/22/2017 5:14:09 PM