Equifax is a genuine credit reference agency. Unlikely they would email you. Look them up and see if they have a facility for you to forward the email.
As everyone has said, they are a bona fide credit checking agency. However, I would be very reluctant to use any information in the letter to contact them (if indeed any contact is necessary) and use the links people have provided to do so.
Do you have an account with any credit checking co?