Attaching BTC to an email is akin to the sender attaching an "importance" score to the email - with the benefit that the sender doesn't like as much because it

Attaching BTC to an email is akin to the sender attaching an "importance" score to the email - with the benefit that the sender doesn't like as much because it costs them money. The receiver can still setup rules - i.e. if the sender is in my family or team at work, don't require the BTC attachment. You could set it up so that it was only for strangers emailing you. Your spam filter would still be in place. Its a way to pay to get a quicker response or more attention - kind of like advertising.