Filter email unless it fits in specific format.
POW(proof of work) is used to ensure the work sender has to do in order to send email.
About POW: here
By applying this rule, wo can tell which senders are determinded. Sender can also mark their email as IMPORTANT by doing this.
To achieve this goal, two basic functionality are required: mining and verifying.
I am not sure how this application can be intergrated in WEB. I am thinking maybe webassembly(which i basically know nothing)?