I am a California lawyer, so don't know the law in Indiana. But as a general rule, truth is a complete defense to defamation. So if everything your ex-lover's wife wrote to your boss is true, defamation going to be a hard road. Maybe other causes of action could apply, like intentional infliction of emotional distress, but I don't think you will find much love from a jury. You would be suing the cheated upon wife for causing YOU pain and suffering, might want to avoid any married women on that jury panel. Don' see a jury giving you money, and losing would cost you thousands.
Defamation relates to false statements of facts. You may have an tortious interference claim. Search Avvo for a lawyer in your city, and call for a free consultation to discuss.
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Even if you do in theory have a legal cause of action, in practice it would probably not be successful if you pursue it. Perhaps a cause of action for invasion of privacy or something like that might work, but again, truth is a defense to any defamation cause of action.