Part of growing is knowing what you did wrong and it’s obvious you don’t understand or get why…. 7 years later you still probably don’t. Whatever you did was worst than not submitting work, if that was the case they would’ve talked to you and dismissed you on good terms. This type of email is only reserved for those who’ve done things along the lines of sexual harassment or breaking the student or graduate or employee code of conduct. Own up to what you did and apologize to everyone involved and then you can cope/grow.