The Dynamic Delivery action in Safe Attachments policies seeks to eliminate any email delivery delays that might be caused by Safe Attachments scanning. The body of the email message is delivered to the recipient with a placeholder for each attachment. The placeholder remains until the attachment is found to be safe, and then the attachment becomes available to open or download.
If an attachment is found to be malicious, the message is quarantined. Only admins (not end-users) can review, release, or delete messages that were quarantined by Safe Attachments scanning. For more information, see Manage quarantined messages and files as an admin.
Most PDFs and Office documents can be previewed in safe mode while Safe Attachments scanning is underway. If an attachment is not compatible with the Dynamic Delivery previewer, the recipients will see a placeholder for the attachment until Safe Attachments scanning is complete.
If you're using a mobile device, and PDFs aren't rendering in the Dynamic Delivery previewer on your mobile device, try opening the message in Outlook on the web (formerly known as Outlook Web App) using your mobile browser.
Here are some considerations for Dynamic Delivery and forwarded messages:
If the forwarded recipient is protected by a Safe Attachments policy that uses the Dynamic Delivery option, then the recipient sees the placeholder, with the ability to preview compatible files.
If the forwarded recipient is not protected by a Safe Attachments policy, the message and attachments will be delivered without any Safe Attachments scanning or attachment placeholders.
There are scenarios where Dynamic Delivery is unable to replace attachments in messages. These scenarios include:
Messages in public folders.
Messages that are routed out of and then back into a user's mailbox using custom rules.
Messages that are moved (automatically or manually) out of cloud mailboxes to other locations, including archive folders.
The user's mailbox search folder is in an error state.
Exchange Online organizations where Exclaimer is enabled. To resolve this, see KB4014438.
S/MIME) encrypted messages.
You configured the Dynamic Delivery action in a Safe Attachments policy, but the recipient doesn't support Dynamic Delivery (for example, the recipient is a mailbox in an on-premises Exchange organization). However, Safe Links in Microsoft Defender for Office 365 is able to scan Office file attachments that contain URLs (depending on how the global settings for Safe Links are configured).