In the message list where the bounce messages are at, select all of the bounce messages. Then, right-click the selection, goto "contact, then "add contact", then the "bounces" folder and then "add contact here".

Then, in the contacts panel, right-click the "bounces" folder, and choose "compose". A compose tab will open up with all of the addresses in the To field separated by commas. You can then ctrl + a in the To field and choose to copy and paste them in a text editor or something. You can then further process them elsewhere as needed. Or, you can just right-click on the "bounces" folder, goto "file" and choose "save selected contacts as" to save them to an adr file. Then, you can open the adr file in a text editor. Another way to do it is to ctrl + left-click the first address in the bounces folder. Then, shift + click the last address in the bounces folder. Then, right-click the selection and choose "copy". Then, when you paste into a text editor, each address will be on a separate line.

In other words, there are a bunch of ways to do it.

If you have trouble filtering a message list, you can create a label under the "Labels" access point, right-click it and goto its properties to create rules to grab all those bounce messages. Then, you'll be able to view only them in that label for easier processing. Just make sure to set the label to filter existing messages too.

When you're done messing around, you can delete that contact folder and delete the messages.

Having said all that, I know my provider doesn't send bounce messages to senders it considers as spam, so expect that there are lots of people who are not seeing your newsletter that you aren't receiving messages.