I'm sorry, I didn't explain myself properly!

I mean to have a mark on column C on the rows where in column A there is a
value that is also in column B (anywhere on column B, not necessarily side by
side)

"frederico roldao" wrote:

I have a sheet with 3 columns.

On column "A" I have 1000 email addresses, on column "B" I have 200 email
addresses.

On column "C" I want to have a formula that will mark the cell if the
address on the adjacent cell (in column B) also exists somewhere in column A.

How do I go about this?