There appears to be nothing unlawful about that comment. If the rest of the communication is the same or less threatening, then I would say you have no claim.
Harassment and retaliation are not, per se, unlawful. Only harassment or retaliation based on an illegal motive are actionable. Illegal motives are motives that are based on discriminatory attitudes toward legally protected classes of people, or motives that are illegal as contrary to specific whistleblower statutes. Unless the harassment directed at you is based on these unlawful motives, then the conduct, while rude, unreasonable or irrational, is not unlawful.
If you believe the conduct is unlawful you should consult with an experienced employment law attorney. I would suggest you look either on this site in the Find a Lawyer section, or go to www.cela.org, the home page for the California Employment Lawyers Association, an organization whose members are dedicated to the representation of employees against their employers.
Good luck to you.
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