Even with single copy + list, with a poll-and-request delivery model (sender polls recipient's system, which then requests message on some schedule):

1. Sender canot simply mass-cram messages to recipients, as many spam engines today do. (Qmail for a time was notorious for doing this).

2. There's good chance that any bulk spammer will accrue a negative reputation by the time most recipients bother to request messages. So they won't.

3. Spammer must sustain a stable online presence --- not be knocked offline or blocked by ISP or hosting provider.

4. Spammer has to track successful deliveries vs. messages that are complete.

Upshot: the spammer's workfactor increases.