Lets say you are talking to a representative of a bank or government institution on the phone, and the representative requests that you send some documents, e.g. document A, B, and C. You ask if you can send the documents by e-mail (method 1), but is told it is not a secure enough method. Alternatively you can log on to the institution's web-site and upload the documents (method 2), or you can send them by physically by mail (method 3).
Is it possible to quantify the risk of these methods somehow? Why would method 1 be less secure than method 2 and 3, when the number and type of documents match what was agreed, and your name is stated on them? Who could possibly interfere the exchange that happens in method 1, compared to method 2 and 3?
Alternatively, why is method 1 considered not to be secure enough?