Mark E-Mails external with Exchange Online External Sender Identification

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For some time now, Exchange Online has offered the option of making external emails visible directly in Outlook as well as OWA and Outlook Mobile. This function is called quite unspectacularly “ExternalInOutlook”. In action the function looks like this:

I have already published an article on the topic of email banners with Exchange Transport Rules online. This feature can replace the transport rule (but doesn’t have to).

Additionally, the following notice will be displayed when opening the email.

Another screenshot inside OWA.

Outlook Mobile.

This function can only be globally enabled or disabled, however, domains or email addresses can be defined as exceptions.

NOTE: a maximum of 50 entries can be defined per tenant! See the link AllowList.

To enable the feature, a connection to Exchange Online PowerShell must be established.
Initially, it can be checked whether the function is active within the tenant.


Enabled is be default „False“ so not active.

for activation you need to execute the following command.

Set-ExternalInOutlook -Enabled $true
##additional allowlist
Set-ExternalInOutlook -Enabled $true -AllowList @{Add="*","",""}

As mentioned above, please refer to the -AllowList parameter.

Additional information on adding domains, etc. can be found at

After activation, it should look like this.


Of course, the transport rule in Exchange can also be deactivated via PowerShell.

Disable-TransportRule -Identity "External-Banner-bottom"

That’s it, good luck with activation and disabeling.

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