You probably have a de-duplication rule set in SFDC that is preventing you from creating records that qualify as duplicates as per the rule set. I'd advise you to check with your SFDC admin regarding the active de-dupe rule set in SFDC. Your SFDC org probably has records with the same email addresses you're trying to create as net new records in SFDC from Marketo (probably the sync user isn't able to see the original record and that's why the record hasn't synced over from SFDC to Marketo in the first place). You could adjust the sync user permission so these people get synced over to Marketo or adjust your dedupe rule in SFDC so Marketo is able to create new records, but fair warning that with the latter option, you'd end up having duplicates in your SFDC org.
... View more