Dynamic Delivery and previewing with ATP Safe Attachments

Basic features of Dynamic Delivery

Dynamic Delivery is an option that can be selected for ATP Safe Attachments. Read this article to learn about Dynamic Delivery and attachment preview capabilities in ATP Safe Attachments in Office 365.

When ATP Safe Attachments policies are set up for your organization, there are several options for how email attachments are handled. These include Block, Replace, and Dynamic Delivery. Depending on how ATP Safe Attachments policies are configured, email recipients might experience a minor delay in email delivery while their attachments are scanned. To avoid message delays, choose Dynamic Delivery.

How Dynamic Delivery works

Dynamic Delivery eliminates email delays by sending the body of an email message through to the recipient with a placeholder for each email attachment. The placeholder remains until a copy of the attachment is scanned and determined to be safe by ATP Safe Attachments.

  • As each attachment is cleared, it is available to open or download.

  • If an attachment is determined to be malicious, it is sent to quarantine, where someone on your organization's security team (such as a global administrator or security administrator) can manage quarantined messages in Office 365.

Most PDFs and Office documents can be previewed in safe mode while ATP scanning is underway. If an attachment is not compatible with the Dynamic Delivery previewer, email recipients see an attachment placeholder until ATP Safe Attachments scanning is complete.


If you're using a mobile device, and PDFs are not rendering in Dynamic Delivery previewer at first, try signing in using your mobile browser.

With Dynamic Delivery, people can read and respond to their email messages right away, while their attachments are being analyzed.

ATP Safe Attachments scanning takes place in the same region where your Microsoft 365 data resides. For more information about data center geography, see Where is your data located?

What happens when someone forwards an email that contains an attachment?

Suppose that an organization is using Dynamic Delivery for their ATP Safe Attachments policy, and someone receives an email that contains an attachment. Now suppose that person forwards the email message to someone else. What happens? It depends on whether the additional recipients are included in ATP Safe Attachments policies.

  • If a recipient is covered by an ATP Safe Attachments policy using the Dynamic Delivery option, then the recipient sees the placeholder, with the ability to preview compatible files.

  • If a recipient is not covered by an ATP Safe Attachments policy, then the email and attachment will go through, without any ATP Safe Attachments scanning or attachment placeholders.

What's required for Dynamic Delivery to work?

Additional considerations

There are certain scenarios in which Dynamic Delivery is not supported. These include the following:

  • Email messages that are in public folders

  • Email messages that are routed out of and then back into the user's mailbox using custom rules

  • Email messages that are moved (automatically or manually) out of the hosted mailbox and into other locations, including archive folders

  • Email messages that are deleted

  • A user's mailbox search folder that is in an error state

  • Environments in which an Exchange Online admin has enabled Exclaimer. To resolve this, see Messages with attachments are not delivered when ATP Dynamic Delivery and Exclaimer are used

  • Messages encrypted with Secure/Multipurpose Internet Mail Extensions (S/MIME))

  • In cases where Dynamic Delivery is not supported, ATP Safe Attachments will not scan email messages. However, delivering email messages with attachments that contain URLs will be checked, depending on how your ATP Safe Links policies are configured. In these cases, URLs in email messages and Office files are checked.