When a message has been forwarded, invalidate the "to" ext/net ptrs
The convenience pointers to_ext and to_net need to be invalidated (nullified) if we are going to allow forwarding from local email boxes to network mail and keep all the original message header fields in tact.
The "Forwarded" field must now be added *before* any new recipient information (local user extension or network address of the forwarding destination).
This is part of the solution to request #83, #108, and #141.
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