Relevant Products: Signature Manager Exchange Edition
You would like to tag incoming messages to state whether it came from an external source.
To achieve this, create a new template and add text to the signature design with the word "External".
Ensure this is above both of the reply separator fields, add any highlighting or font changes that you wish to this text.
It should now appear similar to the below:
Save this template.
You will then need to create a policy to apply this signature, for this the condition should state that the sender is external and the recipient is internal, as shown below:
With both of these configured your external mail should be processed by Signature Manager and have the "External Mail" tag added to the top of any mail coming into your business.