What is a Trade Secret?
It may not have been advisable, but generally, there is nothing illegal about a company revealing the contents of a letter from a member of the public to another. A letter written to a company by a citizen is not classified as confidential, without other factors, such as a confidentiality agreement, involved.
The answer will depend in part on the type of information, whether it was obtained through the course of employment, and whether you have a confidentiality agreement with your company. Under the Uniform Trade Secrets Act, trade secrets are defined as follows:
""Trade secret" means information, including a formula, pattern, compilation, program, device, method, technique or process that:
(1) derives independent economic value, actual or potential, from not being generally known to, and not being easily ascertainable by proper means, by other persons who can obtain economic value from its disclosure or use, and
(2) is the subject of efforts that are reasonable under circumstances to maintain its secrecy."
The Economic Espionage Act of 1996 (EEA). The EEA was in part modeled on The Uniform Trade Secrets Act (UTSA), a model law drafted by the National Conference of Commissioners on Uniform State Laws but expands UTSA's definition. The EEA definition of trade secret follows from Section 1838, paragraph (3):
"[T]he term 'trade secret' means all forms and types of financial, business, scientific, technical, economic, or engineering information, including patterns, plans, compilations, program devices, formulas, designs, prototypes, methods, techniques, processes, procedures, programs, or codes, whether tangible or intangible, and whether or how stored, compiled, or memorialized physically, electronically, graphically, photographically, or in writing if—
"(A) the owner therefore has taken reasonable measures to keep such information secret, and
"(B) the information derives independent economic value, actual or potential, from not being generally known to, and not being readily ascertainable through proper means by, the public[.]"