Hi Devan,
Yes, they can -- the Outlook plug in can't "know" if a lead is unsubscribed in Marketo or SFDC or MS Dynamics. They way it works is, when the user clicks "Send with Marketo", the email is sent and the plug in attempts to append an exisitng lead record with this information by looking for the email address in Marketo (it doesn't, however, search to see if that lead in Marketo has unsubscribed, etc...). If the email address can't be found, then a new lead is created in Marketo, set to unsubscribed, with a strange last name called "Mktounkwn" or something to that effect, with the original source type being "Sales Insight".
Hope that helps,
Jono