Email isn't free. Someone is already paying for it. And most people are receiving far more than they send today, so most of the payments would come from the companies sending emails. They are already paying a lot for email anyway.
Discussion
Your right. Email isnt free.
2 things:
1) consumer email - its difficult to get people to pay for stuff that they have for free now. Not impossible but difficult.
2) commercial email - if companies are alresdy paying for email but spam is alive, then I'm not clear how changing the unit of account to pay in btc helps (unless people deeply understand future value of bitcoin)
To me its more about changing the whole system period. Universal Bitcoin unit of account would stop spam, but it would need to be NOT isolated to email.