You don't have any rights in a physical letter you send away. There would be no issue with the recipient engaging a service to handle letters that happen to be sent to them, and that service processing those letters however they saw fit, including by recording information about the sender(s).

I guess what I'm saying is... what's your point?

edit:

You appear to be misreading your parent comment. The parallel situation they describe is precisely that the recipient engages a company to process their mail (read the description: "you hire some company to sort and collate your mail (and toss out your junk mail for you), and in exchange they slip in some coupons [for] people who get the kind of mail that you do". All of that applies to receiving mail, not sending it.)

And this is precisely what's happening with Google: if you send mail to my gmail address, I've engaged Google to process it on my behalf. You have no say in the matter, nor can you.