Security Policy Errors

How to Fix External Sender Warning in Email (Microsoft 365)

Phi Hung Vo Sep 28, 2026 872 views

Question details

Users sending emails from Google Workspace or other domains are noticing that recipients in Microsoft 365 organizations see an '[External]' warning tag on their messages.

Why Recipients See an External Sender Warning in Email

Product: Microsoft 365 Exchange Online / Google Workspace
Device & OS: not provided
Scenario: Sending email statements or regular correspondence to clients or partners who use Microsoft 365.
Observed behavior: Recipients see a warning tag stating '[External]' or 'This Message Is From an External Sender' at the top of the received emails.

Before you start

Ensure you have access to your domain's DNS settings to verify email authentication records, and note that the external sender warning is primarily controlled by the recipient's IT administrator, not the sender.

Solution 1Recommended

Verify SPF and DKIM Settings for Your Sending Domain

Ensure your Google Workspace domain is properly authenticated to prevent emails from being flagged as suspicious by recipient servers.

While email warnings are mostly controlled by the receiving end, setting up proper domain authentication is the best practice to ensure your emails are trusted.

Check SPF Records

Log into your DNS provider and ensure the TXT record for your Google Workspace domain is correctly formatted, present, and active.

Configure DKIM

Generate a DKIM key in the Google Workspace Admin console under the 'Authenticate email' section and add it to your domain's DNS records.

Allow Time to Propagate

Wait up to 48 hours for DNS changes to take effect globally across all email servers.

Verify SPF and DKIM Settings for Your Sending Domain

Sender Limitations: Proper email authentication improves deliverability, but it will not remove the external tag if the recipient's organization forces it for all outside domains.

Solution 2

Disable External Sender Tagging via Exchange Online (For Recipient Admins)

The recipient's Microsoft 365 administrator can turn off the external warning feature globally using PowerShell.

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Frequently Asked Questions

Can I remove the [External] tag from emails I send to clients?

No, you cannot directly remove it as the sender. The tag is applied by the recipient's email server (like Microsoft 365) as an internal security measure. Only their IT administrator can disable the feature.

Does the external sender warning mean my email is marked as spam?

No, it simply informs the recipient that the email originated from outside their organization to help prevent phishing. It does not negatively impact your spam score as long as your SPF and DKIM records are valid.

Why did this warning start appearing after I switched to Google Workspace?

The timing might be coincidental, or the recipient organization recently enabled the 'Set-ExternalInOutlook' security feature in their Microsoft 365 environment to protect their network from external domains.

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