If I send out an email campaign, I can't use custom http headers to know that a user arrived from the newsletter.
use a unique url for each email
As your reader, I might not actually want you to know.
Do you really need to? Basic statistics will tell you if the email campaign had any significant effect on site visits.
If you are sending out an email, you can use whatever url form you like?
This is talking about links to third party sites, not your own.