Legal Question in Business Law in Massachusetts

Laypeople often use the term "legal document" as if it means something. It doesn't.

Whether a given statement can be used in court will depend upon what it says, what it is being offered to prove and the surrounding context. It usually won't matter whether the statement was made orally, in writing, via email or what have you.

If the approval you describe is relevant to your case and if the email contains a statement that shows the approval, then it will probably be admissible. Whether it actually will be will depend upon additional facts.