External E-Mail tagging in Exchange Online

We all know the e-mail disclamer and “EXTERNAL” tags in subject lines that should make clear, that a specific e-mail is coming from external sender. Mostly this is done to make sure nobody clicks on links in external e-mails that might look like an external e-mail.

This can easily be done by creating a transport rule in Exchange or Exchange Online that matches senders outside the organization. This rule adds something to the beginning of the subject line, and usually a preamble is added to the mail body.

But this approach has one major and one minor disadvantage. In any case, there are two disadvantages.

1. When receiving singed e-mail, chaging the body of the e-mail breaks the signature.
2. Depending on how the rule is build, disclaimer and tags are added to each reply.

Why don’t let Exchange Online tag external e-mails for you?! Outlook on the Web, Outlook 365 Windows, and Outlook for iOS support this, and the feature, which was added in April 2021 (!). To be honest: Outlook 365 Windows supports this sind June 2023.

You can simple enable it using PowerShell using the Set-ExternalInOutlook cmdlet.

PS C:\> Get-ExternalInOutlook | fl enabled,AllowList

Enabled    : False
AllowList  : {}

PS C:\> Set-ExternalInOutlook -Enabled $true

PS C:\> Get-ExternalInOutlook | fl enabled,AllowList

Enabled    : True
AllowList  : {}

It can take 24h to 48h until the first mails are tagged. At some point you should notice this “External” tag in the “from” field.

I definitely prefer this approach against a transport rule.