Is this Libel or defamation?

It would be neither. Libel is false and malicious *published* statements. Defamation is malicious lies intended to damage someone's profession or reputation. Writing them to you in a private email will probably be sufficient for you to win a civil suit because that's called an opinion. If, however, you can produce a witness to whom she also sent this email or repeated the false allegations, and can show that she *knew* them to be false, you might have a chance of success.