Hi Zena:
When you send an email through Sales Insight, and it is sent to someone who is not in Marketo, then all Marketo gets is the email address. That's not enough information to make a meaningful owner assignment in SFDC.
Unfortunately, the way the sales emails are logged, we cannot access the sender information and use it to force a lead owner assignment when the lead is pushed into SFDC.
I can offer you two possible ways to handle this:
1. Ask your salespeople to create a lead in SFDC first, before using the Outlook plug-in to send the first email. This way, the lead could have all relevant info including a lead owner, and it would be pushed into Marketo accordingly. Sales emails sent through Sales Insight should then show-up on the lead's activity log.
2. If you'd rather hold back these leads from being synced into SFDC, you can add filters to your Lead Lifecycle campaign that manages the SFDC Sync. Your filters would ensure that no new lead is synced to SFDC unless it has the information required for a meaningful salesperson (lead owner) assignment. However, in this case you need to ensure that such leads are synced as soon as the required information has been added to them.
Hope this helps at least a little.
Best,
Nicole