some advice i can offer is to have the no email address leads blocked, there really is no use for them in Marketo. You can work with your SFDC Admin to do this on their end, or simply have it blocked by requesting it via support@marketo.com. For the leads you have existing in there that have no email addresses, i would delete those by creating a smart a list, but ensure you don't delete from SFDC 🙂 Best of luck.
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