[fnordfnordfnord]

http://www.nytimes.com/interactive/2013/06/30/us/30postal-ma...

Section I B - allows the contents of unsealed classes of mail "as allowed by law"

Section II B - "...Mail Covers do not authorize the search, seizure, or opening of any class of mail."

Section III B 6 - addresses attorney-client privilege.

[anigbrowl]

None of which is responsive to the question asked above, which was about a letter that was lawfully opened pursuant to a warrant, but which contained a second, sealed letter.