How to explain that a feature is only "partially" secure?

Question

Background
I'm working on a mobile app for an institution that has a sharing functionality for documents that may contain private information. The user can send a document that exists in the app by email. The email content is only a link to a secure storage site hosting the document and is only active for a short period of time, after which the link expires. I need to explain to users that using this function is thus “semi-secure” and we can’t guaratee that no one else can access it (if someone has hacked the recipient’s email, for instance).

Current message copy

Sending by email: AppName hosts your document on a secure site and shares it by emailing a link to that document that expires after 12 hours for security reasons.

Questions

1. I want the (mostly non-technical) users to be able to make an informed choice of whether to use the feature or not to send something private. How can I explain that the storage site is secure but that email is not secure?
2. Should the message appear the first time the user wants to use the feature only?

(Also, any tips on how to get better at writing copy that isn’t clunky would be useful)

Answer 1

For this, I would take a leaf out of Google's book. On Google Drive, when you want to share a document, you are provided with a popup and you get to choose whether you want to allow people with the link to view, comment on, or edit the document (screenshot below).

I'm assuming that level of control is too detailed for your app, but I would simply go with something along the lines of (new sentence in bold):

Sending by email: AppName hosts your document on a secure site and shares it by emailing a link to that document that expires after 12 hours for security reasons. Anybody with access to the link during that time will be able to view the document.

This indicates that it is then up to them to keep the link secure from anyone else, without explicitly stating that you know the method itself is not 100% secure. Because it is also only a small sentence additional to what you already have, the message can be shown every time the user decides to share something, rather than having a one-time alert.

However, as Michael Kohne mentioned in the comments, if you know it's not secure, you may be better off finding an alternative way of sharing the documents if time/budget allows.

Google Drive Share Popup