Why aren't bounced emails being sent to SENDER_EMAIL?

This occurs when you configure WebScriber to use the built-in PHP mail functions. You can check this by looking in the config.txt file: if you see USE_BUILTIN_MAIL=on then you are using the built-in PHP mail functions, and could be affected by this issue.

The only way to make sure the bounced emails go to the SENDER_EMAIL address is to use the SMTP mailing features. Make sure you've set the correct values for SMTP_HOST_NAME, SMTP_USERNAME, SMTP_PASSWORD and SMTP_REALM. Then change the built-in mail setting to USE_BUILTIN_MAIL=off. Any emails that bounce should then go to the SENDER_EMAIL address.