Rush sending is overnight, ala.. if someone needs to send something Dec. 1st, and it's today.. (in the afternoon), then you could have sent it Express and it would have been there by noon tomorrow.
The problem with rushing scores is that some schools will not accept it, or at least strongly discourage it. It requires special handling because they are sent manually in the mail instead of electronically.
I think rush is through (fast) mail. UNC and Harvard, however, are able to access scores electronically, almost instantly, which seems to have been the case in your situation. Strange that schools like MIT do not have the ability to receive scores electronically.
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