The best analogy re: address info on mail is to garbage that you leave on the curb for the trash collector. You probably don't have a reasonable expectation of privacy in that garbage because you exposed it to the public by leaving it out on the curb. There is case law to back this up. Similarly, if you hand over a letter to a postal carrier, you arguably wouldn't have a reasonable expectation of privacy in the info on the envelope since that info can be gleaned by anybody who looks at the letter. Email would be different since that is presumably store on your computer or a server or some other place or thing that would fall under the 4th Amendment protections (and require a warrant).