Field mappings in Salesforce offer the option to retrospectively apply changes. When this option is selected, a batch job is initiated to update Mailchimp records in bulk according to the defined field mappings. More on Retrospectively Apply
Note: Within a single audience, if you have subscribed the same contact or lead with multiple email fields (standard Email and custom emails, like in NPSP), only the record associated with the mapped email field (Salesforce to Mailchimp direction) will be updated in Mailchimp during this batch process.
If you need to update audience records for un-mapped email addresses in bulk, field mappings alone won’t suffice for retrospective updates. Instead, you should use the Data Wizard once to handle updates for these additional email addresses.
