SPF requires the SMTP client IP to match the envelope sender (return-path). When a message is forwarded through an intermediate server, that intermediate server may need to rewrite the return-path to remain SPF compliant. If the message bounces, that intermediate server needs to validate the bounce and forward the bounce to the original sender.
This package contains the srs client that can be used in your .forward files.
More information on this package can be found on the Debian web site
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