Can't get e-Newsletter bounce handling to work

I have installed the plugin and set-up sending to use a smtp server on localhost and an external pop3 mailbox and email address for bounce handling. The host runs a smtp server that is solely used for sending newsletters, while an external mailserver is used for handling pop3, imap and smtp for users of the domain. The connection test completes successfully.

When sending a test newsletter to a test group with valid and invalid email addresses no bounces are sent to the external pop3 mailbox configured for handling bounce emails. The headers of bounced emails don't include the email address for bounces in anyway. This is one problem.

To get bounces sent to the bounce email address I started using sendmail to use the linux user's email (it's a LAMP server) for sending the emails. Hence, bounces are sent back to the linux user's email address that then are forwarded to the email address for bounces but the plugin doesn't seem to to do anything with them. This is indicated with 0 bounces for said test newsletter and no invalid email addresses removed.

Can someone please help me?