Question
What is the "EXPERIMENTAL: Rewrite From: header to match actual sender" option in "Exim Configuration Manager?"
Answer
This setting rewrites the "From" header in emails to show the original identity of the actual sender for messages sent from your server. Email recipients can see the original From header as the "X-From-Rewrite" header as well as the rewritten "From" header. This setting is useful to determine the actual mail sender.
In order to conduct an attack or send unsolicited email, a malicious user can alter the "From" header in an email to confuse the recipient. For example, a user may authenticate as user@example.tld and send a message with the "From" header set to account@domain.tld. When you enable this setting, Exim rewrites the "From" header to show the authenticated sender (user@example.tld).