NOP v 4.2
I tried all of the methods in this thread and none of them worked. Here is what I did and it worked:
1: I exported the existing newsletter subscribers to get the correct format.
2: What did work was I used excel to check remove duplicates
3: Used excel to check for valid email address using this method on this page:....
4: I used excel to combine columns in a row to achieve this format.
5: I pasted 2500 emails into the exported csv file mentioned in step number 1 and saved it as a txt file.
All of them imported.