Can I post in internet a letter my ex domestic partner wrote to himself talking about a fight?

Sometimes the answer to bad speech is more speech, but sometimes restraint is a better idea. He published a letter saying you're a psycho - that's probably hyperbole and not defamation, but if you retaliate by publishing an e-mail he wrote which wasn't written to you, it raises the question of how you got that e-mail and exposes you to the torts of conversion, trespass, violation of privacy, etc., whether the contents are true or not.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.