The latest Gmail web client comes with a plethora of add-ons. One of the most useful is the “Create a document” feature, which essentially turns your email into a Google Document and lets you download it to a local directory (in several formats). You can access and enable this function from the Settings “Labs” tab.
It is enabled by hitting “g” then “w”. Simply select an email and from “More” Create a document…
Now we have the email opened as a Google Document, which we can download to a local directory (in pdf format).
Now our email is stored away as a document and it cannot be tampered with as it’s a pdf.
Lastly, it’s worth noting that the document itself is also stored on-line in your Google Docs screen, which you can keep or bin.