I'm sorry to hear of your dilemma.
What you are describing is known as electronic eavesdropping and it is indeed illegal, under federal law and most state laws. Violation of the law can result in heavy fines and imprisonment up to five years.
18 USC §2511 prohibits the interception of oral, wire, or electronic communications. The statute expressly states that any person who "intentionally" intercepts, endeavors to intercept, procures another person to intercept; or uses, endeavors to use, or procures another person to use a mechanic device to intercept any wire, oral, or electronic communication commits a punishable crime. When a device is used to intercept the electronic or wire communication, that device can be affixed to, or transmit a signal through a wire, cable, or other like connection. Also, when a device is used for the interception of the electronic communications, the person's conduct is also punishable if that person knows or has reason to know that the device has been mailed or transported in interstate foreign commerce. This statutory section sanctions the eavesdropping conduct even if "takes place on the premises of any business or other commercial establishment the operations of which affect interstate or foreign commerce; or obtains or is for the purpose of obtaining information relating to the operations of any business or other commercial establishment the operations of which affect interstate or foreign commerce." It is also a crime to "intentionally" disclosing, or intending to disclose, the intercepting communications to any other person; this is particularly applicable to spouses that intent to use the eavesdropped e-mails in family courts. This crime carries fines or imprisonment for up to five years. In certain circumstances, usually involving more serious violations, the person may be civilly sued by the Federal Government.
I wish you well.
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