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Emails Sent from Sandbox Add the "Sandbox:" prefix to the Subject

Publish Date: Aug 30, 2025
Description
When you send an email from a Case record in a sandbox environment, the "Sandbox:" prefix is automatically added on the subject of the email when sent.

This prefix will always be added whenever sending emails. If the email message the user is replying to does not match the previous email subject, there is a chance the "Sandbox: " prefix will be readded. This may be detected as a separate thread when received by some mail clients as the subjects of the email differ from one another.
Resolution

This is working as expected when sending emails from a sandbox. It is also by design courtesy of the receiving mail client if they categorize the multiple emails sent from Salesforce as different threads if the subjects are not the same.
Knowledge Article Number

000392246

 
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Salesforce Help | Article