By default, Email-to-Case populates the From Address field on the Email Message record using the From value from the incoming email header.
However, if the From value matches an existing Email-to-Case routing address and the email header also contains a Reply-To value, Salesforce populates the From Address field on the Email Message record with the Reply-To value instead of the From value from the email header.
This behavior is working as designed.
This behavior is intentional and helps prevent automated responses from being sent back to the Email-to-Case routing address, which could otherwise result in email loops.
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