ATP attachment scanning is an additional layer of protection specifically for finding and "detonating" malicious content. 

When an attachment is sent to your address, the message will arrive in your inbox approximately 2-5 minutes after sending, depending on the size of the attachment. For example, a 150 KB attachment may take about one minute to send, while a 15 MB attachment may take three minutes.

Once the scan is completed and the attachment is deemed safe, the message will arrive in your inbox and the attachment will be accessible.

If an ATP scan reveals malicious content, it will be stripped and you will see a message similar to the one below.