I understand how frustrated you must be right now. If the email was directed to a third party (e.g. your employer or/and coworkers), then it is an actionable defamation claim. Since the email attacks your ability in your employment duties, you are presumed to be damaged by the statement.
You can hire an attorney and write them a letter demanding to stop making such false claims and/ or sue them for libel. If you ultimately decide to sue them, you can include the original breach of contract claim in your defamation suit as well.