Inter site mail routing of large attachment(s) to multiple recipients

Hopefully this is a really easy one for someone but I am struggling to find any reference to the answer to this...

If a user sends an email with a 10MB attachment to 10 recipients who are all homed on Exchange in a different site(s), does this require 100MB of bandwidth or 10MB? I know there is no single instance storage but will exchange send the attachment once or 10 times?

Thanks

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