Kurt Shintaku's Blog

If you have MailTips enabled for Office 365 on external recipients and receive the following notifications:

EXTERNAL EMAIL : This email originated from outside of organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.

… and would like to add an exception to remove this notification/MailTip for a specific domain so that the “EXTERNAL EMAIL” warning doesn’t display for those domain senders, follow the instructions below using PowerShell to identify an external domain as “internal”:

You can add the specific domain to a remote domain and set it as internal.

New-RemoteDomain -Name <name> -DomainName <domain name>
Set-RemoteDomain -Identity <domain name> -IsInternal $true

And then enable MailTips for external recipients.

Set-Organizationconfig -MailTipsExternalRecipientsTipsEnabled $true