Internal Email Sends, Prevent Lead Creation in our CRM?

Marketo only synchs leads that you explicity tell it to via smart campaigns with flow actions (e.g. Sync Lead to SFDC, Change Owner, etc.) and programs  that are synched to SFDC.  So as long as you flag those lead records such that they will not enter any smart campaigns that have these flow actions or any programs synched to SFDC, they will not be synched to SFDC.

If the program is synched and there is a campaign progression it will force a synch...

We are struggling with some similar issues. One of the things we have done is created an "IsStaff" checkbox that is shared between the systems, so we look at email addresses and other indicators (for contractors or trusted vendors, etc) and set the flag. Then in SFDC we filter out those leads. This way we can still test the flows, but prevent the sales team from seeing junk leads.

Jen what is your use case?  Are you hoping to track opens and clicks?  If not there may be some easier approaches.

Thank you for all the replies, everybody!  Steven, we are not looking to track anything with our internal newsletter.  We deploy it for another department and we are just looking to send a staff-wide email with some newsletter design elements, which is why we wouldn't just send it in Outlook.  Design functionality is all we need though- no back-end measuring necessary.

Jen in that case I'd recommend using a freemium ESP such as Mailchimp.  You'll get the same WYSIWYG functionality to design a nice looking newsletter without cluttering your marketing database.