The third party exception though has come under increasing scrutiny by the courts more recently. The case which really established it in large measure was Cali

The third party exception though has come under increasing scrutiny by the courts more recently. The case which really established it in large measure was California Bankers Association v. Shultz. This is in many ways more important than the pen register case of Smith v. Maryland (http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=us&vol=442&invol=735 http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=us&vo...) because pen registers were held in part valid without a warrant due to their lack of intrusiveness, and in part because the information was not only conveyed to a third party, but also how they were used by a third party. In essence Smith v. Maryland held pen registers allowed without a warrant via "legitimate expectation of privacy" analysis of the sort found in Katz v. United States in part because everyone relied on the phone companies collecting this information and that people relied on the phone companies using this information to be able to track back, for example, harassing phone calls. If individuals can track back incoming harassing phone calls, there is no legitimate expectation of privacy when the police look at those records. The much more concerning line of cases are the banking instrument cases, like US v. Miller (http://caselaw.lp.findlaw.com/cgi-bin/getcase.pl?navby=volpage&court=us&vol=425&page=442#442 http://caselaw.lp.findlaw.com/cgi-bin/getcase.pl?navby=volpa...) and California Bankers Association v. Shultz (http://supreme.justia.com/cases/federal/us/416/21/ http://supreme.justia.com/cases/federal/us/416/21/) which hold no 4th Amendment protection for banking transaction information. The problem with communications is that while call detail records fall outside 4h Amendment protection in general, wiretaps require a warrant (see Katz v. United States) and Smith v. Maryland did have dissenters due to the fact that it is hard to draw a line between these two. More recently a circuit split has developed over whether cell site location data is at least potentially protected. The 5th Circuit (which also holds that all airport searches at security checkpoints are necessarily consented to, so presumably random strip and body cavity searches would be Constitutional) says they are not, while the third says they may be. See http://crimeinthesuites.com/circuit-split-brewing-over-government-access-to-cell-phone-location-data/ http://crimeinthesuites.com/circuit-split-brewing-over-gover... My point is that "knowingly exposed to a third party" is not at all as clear as the page suggests. The reason this line is there is that it is an indication that certain information is not really expected to be secret, but if it was read literally, Katz would have come out very differently and wiretaps would not require a warrant. Obviously if you write details of your criminal conspiracy on the back of a postcard, you can't complain when the postmaster reads it, but when you put it in an envelope then this would be a search. This sort of line is much harder to draw regarding papers outside that sort of home or mail environment than it is in them, but that's largely what I would describe the difficulty to be. This leads to the question of whether email is like a postcard or like a letter or a phone call and I don't think the answer is clear. At any rate I would suggest that this is an area of developing law regarding electronic communications and I have been watching interesting developments there just in the last few years.