Did you ever figure out how to fix this? I'm experiencing a similar problem: when a Sales Rep sends and tracks an email through the Outlook plugin, a new lead is created in Marketo with the first name being the beginning of the email address (ex: aquinn of aquinn@harmonyis.com) and the last name is MktUknown, even though the lead already exists in Salesforce and the email entered was exactly the same as the email of the lead that already exists in the system. We have no idea why this is happening, and it seems to be a new problem. Is this the problem you were having? How did you end up fixing it?
Thanks a lot,
Alex