Can an person be sued for libel by a second party who is insulted in a personal letter from the first party?

Actually, the second party forwarded the first party's email to the third party and the third party is threating to sue the first party for libel and slander. This was a private email from the first party to the second party. The first party stated that a friend was screwed over by the third party in the letter to the second party. Can the first party be sued for libel even though this was a private letter? This was email was not made public by the first party. Please help.