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Understanding DLP: Why Can't I Send This Email?
Applies to: Outlook, Microsoft 365, Teams
Article Type: Informational
Last Updated: 2026-07-10
Summary
You draft a completely normal email to a vendor, attach an Excel file, and click Send. Instantly, the email bounces back with a red warning saying, "Your email was rejected by a Data Loss Prevention (DLP) policy." What happened? Did IT read your email?
This guide explains how DLP works, why it blocks emails, and what you need to do to fix the issue.
What is Data Loss Prevention (DLP)?
DLP is an automated robot that scans every email, attachment, and Teams message leaving the company. It is looking for specific, highly sensitive patterns of text.
IT is not reading your emails. The robot is using algorithms to find things like:
- Social Security Numbers (9 digits formatted a specific way)
- Credit Card Numbers (16 digits passing a mathematical validation check)
- Bank Routing Numbers
- Patient Health Information (HIPAA data)
If the robot detects this data, it immediately blocks the email from leaving the company servers to prevent an accidental data breach.
Prerequisites
- No prerequisites — this is an informational reference guide.
Instructions: How to Resolve a DLP Block
If your email gets blocked, you have a few options depending on what the data actually is.
Scenario 1: The Data is Real (e.g., Actual Social Security Numbers)
If you are trying to email a spreadsheet full of employee social security numbers to a new healthcare vendor, DLP is doing its job. You cannot send that file via normal, unencrypted email.
The Fix:
- Check the rejection email. It often contains instructions (e.g., "To send this data, you must encrypt the email").
- Follow your company's procedure for sending sensitive data. This usually involves clicking the Encrypt button in Outlook before sending, or sharing the file via a secure OneDrive link instead of an attachment.
- Read: How to Encrypt an Email in Outlook
Scenario 2: It is a False Positive
Algorithms aren't perfect. Sometimes, a 9-digit part number (123-45-6789) looks exactly like a Social Security Number to the robot, and it blocks your perfectly safe email.
The Fix:
- In Outlook, you often see a "Policy Tip" warning banner at the top of the email before you even click send.
- Click on the banner or click Show Details.
- You will usually see an option that says Override or Report as false positive.
- Click Override and type a brief justification (e.g., "This is a tractor part number, not an SSN").
- The email will now send normally.
Scenario 3: Accidental Inclusion
Sometimes, you don't even realize the sensitive data is there.
The Fix:
- Check the very bottom of the email chain. Did someone forward you an email from three months ago that happened to contain a credit card number?
- Delete the offending data from the email thread, or remove the attachment, and try sending it again.
Troubleshooting
| Symptom / Error | Potential Cause | Solution |
|---|---|---|
| I don't have an 'Override' button | Strict Policy | For highly sensitive data, IT may disable the override button entirely. You must encrypt the email to send it |
| I can't send a Teams message to a guest | Teams DLP | DLP also scans Teams chats. You cannot paste a credit card number into a chat with an external guest |
| The attachment is password protected but still blocked | Un-scannable files | DLP blocks files it cannot scan (like password-protected ZIP files) because it assumes you might be hiding data. Use OneDrive instead |