Remove duplicates from one excel from another.

I have a master spreadsheet containing my customers data (2701 lines).

I have sent 746 of these customers an email (details in another spreadsheet), I wish these customers to be removed from the master spreadsheet so I am left with the remaining customers who have not been sent the email.

I know it's a VLOOK up which I used to be able to do with my eyes closed many moons ago but cannot for the life of me remember how