Removing duplicates from two different lists

[percyth1]

I have two databases of clients first and last names, List 1 are the clients I want to send emails to and List 2 are the clients I have sent emails to. However, List 1 still contains some clients that have already received emails. I don't know any method of cross referencing list 1 and list 2 to exclude any clients that have already received emails from list 1.

Any help would be appreciated