Microsoft Teams is blocking PDF documents sent from another organisation and flagging them as potential security issue

Hi @Hilton Taylor, 

Thank you for posting your question in the Microsoft Q&A forum. 

This issue is typically caused by Data Loss Prevention (DLP) policies or external sharing restrictions that are configured to block file sharing with specific external domains or organizations. If it only occurs with one client suggests domain-specific or organization-specific blocking policies are in place. To better understand the issue, could you please clarify: 

• Was the file sent as a shared link (like the one shown in the screenshot you mentioned), or was it sent as a regular PDF file attachment? 
• Does this happen with all file types from this client, or is it limited to PDF documents only? 
• Is this issue happening with only one specific external client, while files from other external users are received and opened without any problem? 
• When this client sends the same PDF file/link to other members of your organization, are they also unable to open it, or is the issue only affecting you? 
• Are you the administrator of your Microsoft 365 tenant, or are you an end user? 
• When you click on the 'Know why' link in the screenshot, what information does it show? 

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If you have any updates, please feel free to leave a comment under this answer. Looking forward to your response!

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Hi @Hilton Taylor, 

Thank you for providing those detailed insights. 

Since you're seeing the specific Microsoft Support link about malicious files in SharePoint/OneDrive/Teams, this indicates that your PDFs are being incorrectly flagged as malware by Microsoft’s security systems.

For your reference: Safe Attachments for SharePoint, OneDrive, and Microsoft Teams - Microsoft Defender for Office 365 | Microsoft Learn 

Here are the steps you can apply to your situation (please note: if you're not an admin, you’ll need to contact your Microsoft 365 administrator to perform these actions): 

1. Admin Access to Microsoft Defender Portal 

Navigate to the Microsoft Defender portal at https://security.microsoft.com and check the quarantined files: 

• Go to Email & collaboration → Review → Quarantine → Files tab 
• Look for your blocked PDF files from the specific client. You'll see files with malware detection warnings 

2. Release Files from Quarantine 

You can release quarantined files within 30 days: 

• Select the incorrectly flagged PDF files 
• Click Release to make them accessible again. 
• This provides immediate temporary access while you implement permanent fixes 

3. Submit False Positive Report to Microsoft: 

This is crucial for permanent resolution through Defender Portal. For detailed guidance, refer to Microsoft’s official documentation:   

Resolve False Positive Malware Detections - SharePoint | Microsoft Learn 

Manage submissions - Microsoft Defender for Office 365 | Microsoft Learn 

Manage allows and blocks in the Tenant Allow/Block List - Microsoft Defender for Office 365 | Microsoft Learn 

Thank you again for your time and response. I hope this response brings clarity to the issue and helps you better understand the situation so that communication with your administrator goes more smoothly. If you find the above answer helpful, I would really appreciate it if you could mark the answer as “Accept answer.” 

This will pin the post to the top and make it easier for others with similar issues to find the solution. Your feedback not only helps me improve, but also helps the community grow stronger through shared knowledge. I’m always here if you need any further assistance!