Transport rule to put a header on external email

[Mandi Witkovsky <witkovsm () PFW EDU>]

For those who have a rule set up to add a header to incoming external email, have you seen a decrease in security 
events, or a corresponding increase in awareness?  Did you whitelist any 3rd parties that send on your behalf so that 
the header doesn't appear?  Have you seen any pushback from people?  Thoughts on adding a header vs prepending 
"EXTERNAL" or some such in the subject line?

We're looking into adding this, and I wondered what experience you all have had.

Thanks,
mandi