Defamatory comments

I was summoned by my company without prior knowledge to answer some very defamatory comments made by a "close client" which never had any evidence. These remarks were put in an email to my employing company, some of which went back 2 years. To keep it short, although this happened 18 months ago, I feel very aggrieved and this said person has been repremanded for a similar case  against another worker. I'm still employed with the service company but I think about this frequently; it can't be right. If I follow this up legally is my job at risk?

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