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This filter sorts the legitimate DSN and MDN message from those generated as a result of backscatter. It does this by adding an X-Null-Tag: header containing a computed hash value to every message sent. When a DSN or MDN is received, it contains the original message's header information, which is used to recompute the hash value to be compared against the original. If they match, the DSN or MDN message is accepted, otherwise its rejected or discard according to policy.
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My mail server doesn't do this
