The EventLog shows the error "Message rejected as spam by Content Filtering".
This error message is caused by your Exchange Server and no problem of SmartPOP2Exchange.
Your Exchange Server also includes a simple spam filter module called imf. This one refuses messages detected as spam by default.
Because of this SmartPOP2Exchange saves the email and creates that notification message which is send to the user instead to avoid mail loss.
To solve the problem you need to change the behavior of the Exchange Server and make it accept any message from SmartPOP2Exchange.
You either should disable the content filter and sender filter of your Exchange Server or at least change the action to quarantine instead of refusing.
Please configure your Exchange Server to accept any message from SmartPOP2Exchange. Please note that messages in your POP3/IMAP account already count as finally delivered and because of this the behavior of refusing messages by the Exchange Server is similar to their deletion. If that is your intention we recommend you configure your Exchange Server to delete the messages or move them into quarantine. This avoids the error messages in the log of SmartPOP2Exchange and multiple delivery attempts.