That's a great question. During the initial sync, we rely on email addresses to identify which Emma contacts match your Salesforce leads and contacts. However, once we've mapped email addresses across platforms, an update in a Salesforce record's email address will also be shared with their Emma contact record. This is because both of our services use unique member ID's to identify our contacts, and those ID's stay the same even after an email address is updated.
I hope this helps! Please let me know if you have any questions.