Okay, here's the deal:
1. HR works for the employer. It's sole mission is to ensure that the employer cannot possibly be held liable to any employee under any violation of law. Whenever you think that someone in HR is investigating in order to assist you in your employment, immediately dispose of that thought, because it is utterly incorrect. HR will investigate in order to mitigate damages from terminating your employment, because if the President wants a head to roll, he/she will get one, and HR will do whatever is necessary to protect the employer from the employee's attempts to recover damages.
2. Labor Code 2922 permits an employer to terminate an employee "at will" -- at any time, for any reason, or for no reason at all. Naturally, there are a load of exceptions to the rule, but the rule generally prevails unless the employer is holding a smoking gun while standing over a figurative dead body. The reason is simple: everyone is afraid to risk their job by chalenging the employer's power, and the result is that even when an employee is right, few if any coworkers will ever testify on the employee's behalf.
3. Given that background, I don't know what you've already said to HR, but if you say anything other than that you absolutely did not send any message, and that you're frankly outraged by the employer's investigating you as if you were being made a scapegoat -- then you will
be made a scapegoat and you will be terminated. And, even if you are so unequivocal as to convince HR that you're telling the truth -- you may be terminated anyway, because, Labor Code 2922 permits the employer to do so, absent some proof of unlawful discrimination, or other public policy violation (which doesn't apply to your stated facts).
4. Mainly, HR is looking to find a way to credibly claim that you were insubordinate, because that means if you're terminated, you cannot get unemployment benefits, and that reduces the employer's unemployment benefit contributions. So, once again, if you want those benefits in the event you are terminated, you must be unequivocal that you did not do the dirty deed, and there isn't a shred of evidence that can possibly tie you to that email -- other than you happen to work in the machine's proximity.
5. One last thought: most people want to tell their story, because they feel that justice will be done if they tell the truth. History has proved incontravertably that the more a person discloses, the more evidence they give their opponent, and the opponent will try to connect the dots and make a case, even if there wasn't one. So, when in doubt, you have nothing to say about anything to anyone -- other than that you didn't do it, and you're insulted by the suggestion that you are under suspicion.
Hope this helps.
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