In the summer of 1982, the Israeli army placed a siege on Beirut in a successful attempt to force the PLO out of Lebanon. On August 6, then Chief Rabbi of Israel Rav Shlomo Goren published an article in the newspaper Hatzofeh in which he argued that, according to Jewish law, the siege must allow terrorists to escape the city. Understandably, this caused a bit of a furor and Rav Shaul Yisraeli wrote a letter in response. Rav Goren responded in turn and the exchange was published in Hatzofeh on Sep. 17th. The next year, Rav Yisraeli published an article on the subject in the journal Techumin (vol. 4).
One issue that Rav Yisraeli addressed is that his letter was intended to be a private note to his friend, Rav Goren. However, Rav Goren wrote a response and forwarded both the letter and the response to the newspaper for publication. Rav Yisraeli objected to the publication of his private letter. Rav Goren later said that he had asked the newspaper to obtain permission from Rav Yisraeli before publishing his letter, but that never happened and Rav Yisraeli was upset. Rav Goren then wrote a letter apologizing but arguing that even though he would ask for permission, it was not necessarily halachically required. Rav Yisraeli then published that letter (presumably with permission) along with a response in the journal Techumin, vol. 4 pp. 354-360. Here are some of the sources and arguments.
Rav Goren begins with two texts that seems to contradict each other:
1. The Gemara in Yoma (4b) learns that you may not repeat something that you hear because it says (Lev. 1:1), “Now the Lord called to Moses, and spoke to him from the tabernacle of meeting, saying.” The last word — “saying” (leimor) — seems redundant because the next verse begins with “speak” (dabeir). The Gemara explains that “leimor” teaches us that you have to receive explicit permission in order to repeat something you are told. This is quoted in the Magen Avraham (156:2). Presumably, one would therefore not be allowed to publish a letter without permission.
2. However, the Tosefta (Bava Kamma ch. 7) states that someone who “steals” (overhears) someone else’s teachings may go and repeat the teachings. The Shach (Yoreh De’ah 292:35) rules, based on this, that you may copy Torah insights from someone else’s book even if he doesn’t want you to do so. Therefore, it would seem that one would be allowed to publish a letter of Torah insights without permission.
To explain the contradiction between the above two sources, Rav Goren posits that the Gemara in Yoma was referring to non-Torah related material while the Tosefta deals with Torah. You need permission to tell someone general information you hear. However, Torah belongs to the entire Jewish people. It is something for which we all have a mitzvah to learn and teach. You do not need permission to repeat a Torah insight that you hear from someone else – in fact, doing so is a mitzvah. Therefore, technically you are allowed to publish a letter of Torah insights without permission. However, as a matter of politeness, it is always best to obtain explicit permission.
Rav Yisraeli disagrees with this analysis. He responds that the contradiction can be explained based on the rule that one must review one’s Torah thoughts multiple times before teaching them in public, to ensure that they are properly thought out and appropriately worded. While Rav Yisraeli does not quote this saying, it is appropriate: “Not everything that is thought should be said; not everything that is said should be written; and not everything written should be published.”
According to this understanding, if you tell someone privately a Torah insight he may not repeat it without permission because it might not be sufficiently well thought out for public consumption. Hence, the Gemara in Yoma which forbids revealing what is told without permission. However, an individual may intentionally overhear (“steal”) a Torah insight that is not ready for the public, if he himself will also not reveal it to the public. Either way, a letter that was written for an individual should not be shared with the public because the thoughts might be insufficiently worked out or not worded optimally.