[abcabc321321123]

That is correct, but the warrant requirement does not follow common sense. Let me try to explain further with an example. A sends a letter to B with information in the letter that implicates C in a crime. The FBI does not have a warrant to open and search the letter. The FBI goes ahead and searches it anyway. Now, the FBI has evidence that C was involved in a crime. C, obviously, would like to challenge the search based on the warrant requirement of the 4th Amendment. It is here where the warrant requirement diverges from common sense. C does not have "standing", which is a legal term of art that basically means the court can hear and decide the case, to challenge the search because the mail was not his. Therefore, without standing, C cannot challenge the search even though the FBI did not have a warrant for the initial search. In contrast, A would have standing because he has a "reasonable expectation of privacy" in his own mail (assuming there was incriminating information against A in the letter).

[ewoodrich]

No, that evidence would still be inadmissible. They could use "parallel construction" to obtain new and admissible evidence to implicate C in the crime, and could use the inadmissible information to launch an investigation they might otherwise not have. But the standing issue in this case is that Ulbricht cannot argue that his fourth amendment rights were violated by search and seizure of a third party's property.

39 U.S.C. § 4057 specifically requires a warrant for certain classes of mail and parcels, and therefore any evidence obtained in a warrantless search would be inadmissible.

[abcabc321321123]

Yes, that statute requires a warrant. However, the exclusionary rule is only applicable to one with a "reasonable expectation of privacy." In the hypothetical, C does not have such an expectation, and therefore C cannot challenge the admissibility of the evidence notwithstanding the statute.

[ewoodrich]

Yes, you are correct. Upon rereading your hypothetical, I realized I interpreted it incorrectly.

Of course, it doesn't mean that such evidence would be especially compelling to a judge or jury in those circumstances, but that's a separate matter.