You need to resolve the email delivery delay issue.
What should you do?
From the Security & Compliance admin center, modify the safe attachments policy
From the Security & Compliance admin center, create a supervision policy.
From the Exchange admin center in Exchange Online, modify the antimalware policy.
From the Exchange admin center in Exchange Online, modify the spam filter policy.
References:
https://docs.microsoft.com/en-us/office365/securitycompliance/dynamic-delivery-and-previewing
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