Put the CSV in a data frame and add a “marking” column to the data frame. Add a 1 or an x to the correct row in the data frame anytime the email is used. You can use the index to know how to mark the x. Ie you are in column Email index 5, you then would append an X in column “indicator” index 5.
x = 0
for x in range (y):
df.insert(x, “indicator”, “x”
When you’re done export to a csv and overwrite the pervious file.