Does Microsoft Defender Dynamic Delivery Have a File-Size Limit?
Question details
The user is asking whether Microsoft Defender for Office 365 enforces a specific file-size limit when using the Dynamic Delivery feature for scanning email attachments.
- Product
- Microsoft Defender for Office 365
- Device & OS
- not provided
- Scenario
- Sending or receiving emails containing large attachments that undergo dynamic delivery security scanning.
- Observed behavior
- Dynamic Delivery does not publish a standalone file-size limit. Instead, it inherits the general Microsoft 365 message and attachment size limits, which can result in rejections or processing delays for oversized files.
Ensure you have Exchange Administrator permissions in your Microsoft 365 tenant to view message size restrictions and run message traces.
Review Microsoft 365 Subscription Message Limits
Check your organization's default message limits, as Dynamic Delivery inherits these constraints to determine if an attachment can be processed.
Since Microsoft Defender for Office 365 does not have a unique limit for Dynamic Delivery, the maximum allowed attachment size depends entirely on your specific Microsoft 365 subscription and how your Exchange admin has configured mail flow settings.
Log in to the Microsoft 365 admin center with administrator credentials and navigate to the Exchange admin center.
In the left navigation pane, go to Recipients and click on Mailboxes to view user settings.
Select a mailbox, open its properties, and navigate to the Mailbox features tab. Under Message Size Restrictions, click View details to confirm the maximum sent and received message sizes.
Run a Message Trace for Delayed Attachments
Use the message trace tool to determine if a large attachment was rejected, deferred, or delayed due to Dynamic Delivery scanning.
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Frequently Asked Questions
Why are my emails with attachments delayed when using Dynamic Delivery?
Dynamic Delivery scans attachments in an isolated sandbox environment before releasing them to the recipient. Larger or highly complex files require more intensive security processing, which naturally extends the time it takes to finalize delivery.
How large of an attachment can I send in Microsoft 365?
The default maximum message size in Microsoft 365 typically ranges from 25 MB to 150 MB, depending on your specific subscription tier and how your Exchange administrator has configured your organization's mail flow settings.
Can I bypass Dynamic Delivery scanning for specific large files?
Yes, Exchange administrators can configure Safe Attachments policies within the Microsoft 365 Defender portal to bypass scanning for specific trusted senders or via custom mail flow rules, though this bypasses essential malware protection.
