We are using Microsoft 365 - Outlook.
Recently we are facing issues like email send from many of customers are seems to be from different email users may be hacker.
Like earlier original mail from one customer email@example.com
and now i checked its from firstname.lastname@example.org
. when we send proforma invoice to our customer they receive with different bank details than us due to this same one of customer transferred the payment to this fraudster. How can we resolve this.