Email Security Compliance options modify the header of an email to show the Salesforce bounce syntax to the sender's email address. The emails aren't returned to the original sender's email address but to the Salesforce bounce address in the header.
Here, Salesforce composes a mailer-daemon response to the original sender's email address. Without bounce management activated and "Return bounced emails to sender" checked, end users sending emails don't get their bounces to their email address.
To make sure undelivered emails sent from Salesforce to Contacts, Leads, and others come back to your own email address, choose one of the following two configuration options in Salesforce email setup.
In Lightning Experience: Gear icon | Setup | Email | Deliverability
In Salesforce Classic: Setup | Email Administration | Deliverability
Bounce management is not used with Send Through External Email Services.
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