Short answer is, yes, it is possible for emails to come directly to the special email to case addresses, if that is where the emails are being sent to. A good practice is to have all emails come to your Exchange server, then forward them to Netsuite’s email to case addresses instead. This lets you control which emails go through with rules in the Exchange server.
We have dealt with this before in both our Netsuite and Salesforce instance, and it’s one reason I am not a fan of auto-reply emails, especially when they come from a system that can generate new cases which then automatically reply, and you can easily get stuck in an endless loop like this when the other sender has the same setup.