When you sign email sent by Salesforce with a DomainKeys Identified Mail (DKIM) key, the signature configuration now includes the Reply-To header. To protect against conversation hijacking, this change prevents modification of the Reply-To address during delivery. If you use DKIM keys, you get this benefit automatically. Otherwise, set up DKIM keys to sign outgoing email sent by Salesforce.
Where: This change applies to Lightning Experience and Salesforce Classic in all editions except Salesforce Free Suite and Database.com.