So, each column is a month, and in each column there are the emails of who opened the email?
If you send the emails out to the same group of people each month, I would just use COUNTIF. So, for instance, add a column after the last one that contains all the emails you've sent the messages to, and then do a COUNTIF on the range containing the emails.
A B C D E F
1 Month1 Month2 Month3 Month4 Emails Formula
2 firstname.lastname@example.org email@example.com firstname.lastname@example.org email@example.com firstname.lastname@example.org =COUNTIF(A2:D7,E2)
3 email@example.com firstname.lastname@example.org email@example.com firstname.lastname@example.org email@example.com =COUNTIF(A2:D7,E3)
4 firstname.lastname@example.org email@example.com firstname.lastname@example.org email@example.com firstname.lastname@example.org =COUNTIF(A2:D7,E4)
5 email@example.com firstname.lastname@example.org email@example.com firstname.lastname@example.org =COUNTIF(A2:D7,E5)
6 email@example.com =COUNTIF(A2:D7,E6)
7 firstname.lastname@example.org =COUNTIF(A2:D7,E7)
The same basic principle can apply to finding out how many answered on a specific month; just change the range in the COUNTIF from A2:D7 (or whatever) to A2:A7, B2:B7, and so on.
The only downside here is that you need to keep track of all the emails that you've used for newsletters, so you can put them in Column E. However, hopefully that's not too hard, since you can just copy and paste them from the email you sent. If you end up with duplicates, you can use the "remove duplicates" feature in Excel. HTH!