can't import email addresses

can't import email addresses

I don't understand what the format needs to be in order to import contacts. I have tried adding a header at the top that says:
first name,last name,email address
but that didn't work. The file I want to import just has those 3 fields. But every time I try to import that file it doesn't work. I have tried .csv and xls and xlsx. None worked.

What should the header say exactly in order for this to work. Also when I tried selecting the items in the map these fields page that didn't work either.

If there are specific field names that would work, what are they?? And if that's not the case then please tell me how to import contacts?