I think you can give the user the option to specify the email hash (MD5 maybe) instead of clear text email.

so either:

<form action="//api.formspree.com/user@example.com">

OR

<form action="//api.formspree.com/b58996c504c5638798eb6b511e6f49af">

You can provide the user a small tool to generate the email hash.

Good luck