There is not currently an option in the interface to change this if you have specified that your answer be remembered. However, you can change this value manually by editing the ini file. Here's how:

1. Exit POP Peeper
2. Find and edit the ini file in a text editor (use this faq to find the ini file)
3. Search for AddrBookAutoAdd
4. To prevent addresses from being automatically added, change the value to 0 (ie. AddrBookAutoAdd = 0). To automatically add addresses, change the value to 1. If you want POP Peeper to ask you what to do, delete the entire line.
5. Save the ini file
6. Restart POP Peeper