Warning message when external mail arrives

I hope everybody is starting a great 2024!
I have a UCS 5.0.3, planning to upgrade to newer versions soon. Maybe next week we’ll start with the upgrade plans.
I use OX for webmail, then postfix generally speaking, and noticed that some companies add a warning message when receiving mail from external domains, sumething like this:

“ATTENTION: This email is from outside our domain. Please, do not open attachments if you are not sure they are really sent to you, and do not click on links before checking where they are pointing”

It looks great, and puts some attention when a user receives some fake emails.
Is there any way to add this kind of messages on top of the emails received by the UCS server, and not being sent by the server itself?
Thanks a lot in advance for your help!
Best regards,