MC4SF_MISSING_EMAIL_FIELD

The Salesforce email field was not specified when attempting to push a Salesforce record to Mailchimp, e.g. if you are pushing a Contact record from Salesforce to Mailchimp, check that you have the Contact email field mapped on the Field mappings screen (either the Data Wizard mappings or the generic Field mappings screen, in the direction of Salesforce to Mailchimp).