When spammers sign up for your website, you get a lot of junk email addresses. If you then institute a newsletter module, you may get several hundred “mail delivery failed” messages.
In Apple mail, you can select all the individual messages and click “forward” to compile them into a single email. (If you use conversations, first do View->Expand All Conversations in order to select individual messages.) Then copy that, paste into http://eel.surf7.net.my/, and you’ll get a list of the email addresses that bounced, which you can then bulk unsubscribe. This works for me with the Drupal Simplenews module.