Two years ago, I posted a blog on how to implement Transport Rules in Microsoft Exchange to flag messages originating from outside the organization. Goal is to aid end users in identifying messages originating from outside of the organization, by displaying tags in the subject or body part of received messages. This to make them aware – and hopefully more cautious – when it comes to clicking links or opening attachments. Downside of this method is that every inbound message gets a bit cluttered in their subject or body with tags and notifications, which becomes more evident when replying back and forth to external messages. Back then, I already stated a sort of MailTip would be a more preferable and elegant solution.

Onward to 2021, where tagging of external messages became a generally available feature in March for Exchange Online (MC243047, announcement), when used together with Outlook for Web Access (OWA), Outlook for Mac, Outlook Mobile. Outlook for Desktop will also receive support for the feature (supported per version 2105 build 14026.20000, InsiderFast currently). To start adopting this tagging mechanism for new messages, organizations need to deploy an organization level setting using Set-ExternalInOutlook, e.g.

Set-ExternalInOutlook -Enabled $True -AllowList 'contoso.com'

This will enable the tagging of external messages in your tenant, except for domains or e-mail addresses which have been specified through the AllowList. In the example above, messages from contoso.com senders will bypass tagging. The AllowList is limited to 30 entries or 1 kB, whichever comes first. You can reconfigure the AllowList through the hashtable method, e.g.

Set-ExternalInOutlook –AllowList @{Add='fabrikam.com','john@wingtiptoys.com'; Remove='contoso.com'}

After configuring Set-ExternalInOutlook, tagging is not immediate and can take a short while to become active. To inspect current settings, run Get-ExternalInOutlook.

How tagged mail is presented depends on the client. For example, Outlook for Web Access displays an ‘External’ label in the message list, as well as a MailTip at the top of the e-mail contents:

Same goes for Outlook Mobile, where the message list as well as the message view will show an indicator:

Outlook for Desktop
However, Outlook for Desktop does not present a label in the message list, nor exposes a field to filter those external messages, only displaying a MailTip after opening the message:

So, people almost started asking right away if it was not possible to expose External information in the message list. Well, with a little help of “Oldskool” Outlook and forms customization, this is possible, and here is how:

First, we need information related to the MAPI attribute. As the field is not by default available in Outlook, we need to know some of its properties to define it later on. As mention in some of the documents, or Glen Scales’ article on how to identify messages using Graph or Exchange Web Services, the MAPI property tag is 41F28F13-83F4-4114-A584-EEDB5A6B0BFF and its name is IsExternalSender.

Next, we need to construct a .CFG file where we will define the property we want to expose. I’ve already done this part for you, and you can download IsExternalSender.cfg from GitHub. Also download the two .ico files and put them in the same folder as the .cfg file. Note that those .ico files only represent the form. Alternatively, you can copy the .cfg file to your Personal Forms Library folder and skip installing, but this way the instruction is a bit simpler, allows you to pick the Folder Forms Library and will skip the elevated access dialog as the Personal Forms Library is a protected folder.

Open Outlook, go to File > Options > Advanced and click the Custom Forms button. There, click Manage Forms. By default, the form is installed in your Personal Folder Library. You can also pick a mailbox to store the form, allowing it to roam together with the view changes we will perform later. Click Install and pick the prepared .cfg file you downloaded from GitHub, and the following dialog should be shown:

Click OK to confirm you want to load the information in the form file in your personal forms library and close the Forms Manager. Going back to the Outlook navigation view, you can now add an IsExternalSender field to the message list. Right-click the field header and select Field Chooser. In the drop-down list, select Forms.., and add External Tagging Form to the Selected Forms. Field Chooser should now display an available field named IsExternalSender, which you can add to your current view using drag & drop.

Note that the .cfg defines the IsExternalSender as Boolean and showing it as an Icon. This means that for External messages, the column IsExternalSender will contain a checkbox:

When you want, you can create custom fields to adjust how the information is presented. For example, you can create a custom field using a formula to display [EXTERNAL] for IsExternalSender messages, which might be more usable in certain views instead of the checkbox. To accomplish this, select New in Field Chooser,and create a field named ExternalTag, type Formula and enter the following formula:

iif([IsExternalSender]=-1,"[EXTERNAL]","")

You can then add the ExternalTag field to Compact View. Do note the text takes up a row in Compact view, thus might replace sender or the subject depending on layout and field order.

On a final note, when wanted you can filter, sort or create Search Folders using the new IsExternalSender field.

Exchange On-Premises
Organizations running Exchange Hybrid, routing inbound messages through Exchange Online, are not able to benefit from external e-mail tagging. IsSenderExternal is only stamped on messages destined for mailboxes in Exchange Online. These organizations have therefor no way to identify these messages landing in their Exchange on-premises environment, and may require them to deploy the less elegant Transport Rules solution regardless.