Gross negligence? No. The relations between employer and employee are fundamentally contractual. Negligence doesn't enter into it. At-will employees may be fired for any reason or no reason, unlawful discrimination excepted. Retail employees, for example, get fired all the time for thefts they didn't actually commit, but that employer believed they committed. Employer doesn't have to be RIGHT in such a circumstance to fire its at-will employee. Where an employee is contractually entitled to hold a job unless just cause exists for dismissal, there may be a triable issue about whether there was just cause for dismissal, where employee didn't commit the act alleged to have given rise to just cause. But in the circumstance of at-will employment, liability of the employer for negligence is simply not at issue.
Not legal advice, just my two cents. I don't practice law in Pennsylvania or hold licensure there. If you need legal advice, please consult Pennsylvania counsel. I practice in Vermont ONLY.
Maybe the company didn't do anything actionable, but what about the person or persons who sent the emails? I'm just throwing out ideas here that are outside of my areas of practice, but I would look into whether the person who did this violated a state or federal law having to do with cybercrime. Also, depending on what kinds of emails were sent out, there might be a civil action based on the content of the emails. You might want to re-post your question under the categories of cyber-crime, computer crimes, something of that nature, and possibly tort, and see what kind of advice you get. I'm really sorry this happened to you. It's clearly one of those times in life when things aren't fair.
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Not negligence but the Plaintiff could argue recklessness in not ascertaining the true culprit. on the other hand, the culprits actions were an intervening criminal act, fraud, and possibly relieves the company of liability. I am not an employment lawyer, and you should consult with one.