Comment by angry_octet

If it keeps the original email then the DKIM signature from the sending domain will still be intact. As people have pointed out, it might be necessary to keep a copy of the public keys of all the senders in order to perform that validation.

Frustratingly, Exchange breaks this possibility immediately, because it decomposes the email into components. It does preserve and validate some fancier signature schemes, but those are rarely used and have their own problems.