import csv

a = set()
with open('a.csv', 'rb') as csvfile:
    firstCSV = csv.reader(csvfile, delimiter=',', quotechar='|')
    for r in firstCSV:
        a.add((r[0]))   # use different index for email address column

b = set()
with open('b.csv', 'rb') as csvfile:
    secondCSV = csv.reader(csvfile, delimiter=',', quotechar='|')
    for r in secondCSV:
        b.add((r[0]))

intersection = a & b # use different logic operator if needed

for x in intersection:
    print "".join(x)