MC4SF_MEMBER_EMAIL_CONFLICT

What this means: An email change on a Salesforce record could not be applied to its Mailchimp member because that email address already belongs to another member in the same audience. The member keeps its existing email address.

Common causes: Merging duplicate Contacts or Leads that had different email addresses; changing a Salesforce email to one that is already in the audience.

What to do: Check the Maintenance message for the email address and audience. If both members are the same person, archive the old member in Mailchimp. If not, correct the email on the Salesforce record.