If a new Lead record is created in Salesforce without an email address and the Salesforce connector is set to "automatically create prospects in Marketing Cloud Account Engagement", no new prospect will be created in Marketing Cloud Account Engagement. This is because email address is a required field for creating a prospect in Marketing Cloud Account Engagement, and without it, the sync will fail. The Lead record will remain in Salesforce, but it will not have a corresponding prospect in Marketing Cloud Account Engagement1. Reference: Marketing Cloud Account Engagement Sync Behavior