It's a murky world TinSolier. I found :- it definitely would be a breach of copyright for a friend to publish the letters that you had sent him or her, and you could seek an injunction to prevent such publication, or damages if the publication had already taken place. The friend would have a defence, however, if he or she could prove that you had given 'licence' (permission). Such permission may be explicit or implied.
Also >>> it's worth noting that although the copyright belongs to the writer of the letter, the letter itself - the physical object - belongs to the recipient. So if I have an autographed letter from Margaret Thatcher, I can sell it, lend it for public exhibition, etc. But I can't publish the contents or allow anyone else to do so.