The Battle Creek Enquirer reports the woman told police she believed the peanut butter on a bill was an attempt to harm her.
The case is being reviewed by the Calhoun County prosecutor's office.
The Battle Creek woman told police said she came home during her lunch break Friday and found an envelope in the mailbox addressed to her husband with oil soaking through. The outside of the envelope had writing indicating that it contained peanut butter.
The ex-wife told police she didn't mean any harm. She told police it was meant to keep the woman from reading mail that didn't involve her.