... Nevertheless, I wouldn't feel comfortable in prosecuting a tort action where the writing was not intended for general publication. ...
Certainly if a letter-sender sends a letter containing false statements solely to the individual about whom the false statements were made, the recipient of the letter will have more of a
challenge in a defamation suit trying to claim third-party publication if others happen to read the personal correspondence - and when aspects of a suit are challenging, the costs to pursue the action tend to increase substantially. But (theoretically at any rate), there are many situations where such a claim could be successful (e.g., knowledge that all correspondence is screened by another, the recipient is illiterate or blind, the letter needs translation, foreseeable compelled self-publication ...).
Pennsylvania has not ruled out (entirely) the idea of
compelled self-publication as third-party publication for which the originator of defamatory statements could be held liable, although many states have. You can read Part B, State Defamation Claims, in the case
Susan Leslie Frank v. Allstate Insurance Company, et al (
https://www.paed.uscourts.gov/documents/opinions/14D0571P.pdf) where the Court quotes from
Yetter v. Ward Trucking Corp., 585 A.2d 1022 (Pa. Super. 1991): "We express no view as to whether under a different set of circumstances, compelled self-publication of the defamatory material by the defamed person, rather than by the defendant to a third-party, will suffice."
When the letter-sender makes defamatory statements in a letter, addressed specifically to the person defamed, and the letter-sender has reason to believe the recipient will have to disclose its defamatory contents to a third person (for example, to a prospective employer on stated reasons by former employer for termination), then the letter-sender could be found responsible for the compelled third-party publication.
Here are a few cases to read, if you are so inclined
, on third-party publication when the third-party came upon the defamatory statements from someone other than the originator:
Barnes v. Clayton House Motel, 435 S.W. 2d616 (Tex 1968):
http://www.leagle.com/decision/19681051435SW2d616_1959.xml/BARNES v. CLAYTON HOUSE MOTEL
"If one sends a libelous statement through the mails, addressed to the person defamed, with the expectation or intention that it will be read by another person as a matter of course, and such other person so reads it, there is publication, but where the sender is 'not reasonably chargeable with knowledge that a third person might 'intercept' and read the libelous matter before it reached the person allegedly defamed,' there is no publication."
[/Churchey v. Adolph Coors Co., 759 P.2d 1336 (Colo. 1988) (see Part B, Publication):
http://www.leagle.com/decision/19882095759P2d1336_12027.xml/CHURCHEY v. ADOLPH COORS CO.
"If a reasonable person would recognize that an act creates an unreasonable risk that the defamatory matter will be communicated to a third party, the conduct becomes a negligent communication. A negligent communication amounts to a publication just as effectively as an intentional communication."
K-Mart Corporation v. Washington, 866 P.2d 274 (Nev. 1993):
http://www.leagle.com/decision/19931140866P2d274_11135.xml/K-MART CORPORATION v. WASHINGTON
"Words or conduct or the combination of words or conduct can communicate defamation."