|The law firm is indeed legitimate; the email is a scam. The scammer has hijacked the law firm to give cover to the scam. It is not the only firm being hijacked in this manner. It is becoming fairly common.|
Simple remedy on receipt of one of these "phishy" emails is to look up the firm on the internet. (Do NOT rely on any phone number which may be in the email.) Call the firm and ask for the person who sent the email. (Sometimes the scammer will actually use the name of a real person from the hijacked firm.) Confirm the veracity/scam of the signing request.
WARNING: DO NOT CLICK ANY LINKS IN THE EMAIL PRIOR TO VETTED PERSON-TO-PERSON CONFIRMATION.
And beware of upgraded sophistication of these scams as time passes.