I can't think of any way installation of ListMailPRO would be related to incoming traffic or links from unknown sites.
There are no known vulnerabilities in the latest version available from the members area.
I don't see how any SPAM or messages sent by ListMailPRO could cause such traffic. Typically when people spam they send email with links to their own sites, not the site they're spamming from.. If someone managed to break into ListMailPRO and was using it to send email you should see some evidence of their activity, such as users on lists, messages in Sent Messages, bounced messages coming from your server, spam reports against your IP, etc. Make sure you have a strong admin password and enable the option "Notify admin when a bad password is entered" on the Configuration page for best results. It's also a good idea to have a strong FTP password as dictionary or brute force attacks against those are a common way in for hackers.
If it's just traffic you're seeing I would say it's most likely harmless. Perhaps if you look at the external pages linking to your site you will find the cause or reason.
I have a fair amount of experience on the internet, so if you want hands-on assistance you could order General Support from
http://listmailpro.com/support and I would spend some time investigating and do my best to explain what's going on, but I can't guarantee anything can be done because it sounds like it's the activity of a third party site we have no control over.
Regards