To enhance the security you can burn in the receivers E-mail address into the proxy preview. This is not an over lay but a real burn in, so you will need expect a little delay before receiving the share E-mail as ioGates do not mail the link before the proxy preview is ready. As a guideline you can estimate that the delay is about the duration of your clip and remember that is you have 20 clips and you need to E-mail to 10 receivers, then ioGates need to create 200 proxy previews.
How to use Personalized Watermark. It 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, then all the your receivers will have their E-mail burn in into the proxy preview.
Link to Two factor authentication
Link to Digital Rights Management (DRM)