If I attach something to a mail which I send to 100 people and then each of them might then forward it again somewhere, how many copies of the attachment are the on the servers?
When would a new copy be created?
New copies are created once the email is sent. The file(s) only exists on the server until the email is actually sent. That's why you can get bounceback messages because an email is too large in size from recipients. The attachments are actually embedded within the email itself when it's sent (and forwarded).