A customer of mine is looking for a way to add a Global Signature on each incoming message. For example:
"THIS IS AN EXTERNAL EMAIL. For the security of xxxx, please use caution and do not click links or open attachments from unknown senders or unexpected emails. If you have questions regarding the safety of an email, please contact the Help Desk."
The customer noticed that other companies they communicate with are doing similar things (but presumably not using GroupWise). Also, this is something that Exchange can do: https://technet.microsoft.com/en-us/library/dn600323(v=exchg.150).aspx
One obvious advantage to doing this would be to help combat spoofed messages that appear to be coming from internal users but originate from an outside and/or malicious source. This would let them know it was not sent internally.