Office 365 ATP attachments taking too long for scanning and won't open

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Attachments is taking too long for scanning with ATP attachments enabled in office 365. Why and how to fix this? Is there way to exclude from scanning?
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Commented:
You can control the policy settings as detailed here: https://docs.microsoft.com/en-us/office365/securitycompliance/set-up-atp-safe-attachments-policies#step-3-learn-about-atp-safe-attachments-policy-options

More specifically, you can either disable it completely, enable dynamic delivery (which will deliver the message and replace attachments once they are scanned) or configure the "Apply the above selection if malware scanning for attachments times out or error occurs" setting to ensure that messages are delivered even if the default 1h period expires.

Lastly, if you want to configure a bypass, you can set up a transport rule to add the X-MS-Exchange-Organization-SkipSafeAttachmentProcessing header to any messages you want to bypass the scan, with value set to 1.

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Commented:
When the user try open attachments the scanning takes like more than 1 hour to open, how to prevent this? Dynamic delivery is turn on.
Matt FieldsPremier Field Engineer

Commented:
Nick,
         Im a little confused here, ATP's safe attachments policy scans attachments in transit, not upon opening. Can you share an example of exactly what happens when an end user tries to open an attachment that's been scanned by ATP?

         As mentioned by Vasil, you can change the safe attachments policy to block or replace the attachment based on scanning and the end user would not even have the message that's being scanned.

         Microsoft's SLA is 30 minutes for ATP scanning but over 95% of all messages are scanned in less than a minute so it would be nice to have a better picture of exactly what's happening.

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