To enhance security, you can burn in the receiver's E-mail address into the proxy preview. This is not an overlay but a real burn-in, so you will need to expect a little delay before receiving the share E-mail as ioGates do not mail the link before the proxy preview is ready.
Personalized Watermark is an option under the sharing menu. Select the file or folder you would like to share, and then go to the sharing menu. In the share options, you just select the Personalized Watermark checkbox, and then all your receivers will have their E-mail burn-in into the proxy preview.