Comment by lifthrasiir

CF's success (especially in the free plan) suggests that this might be actually true---I'm afraid I cannot prove or disprove the claim (that's why I believe so). My observation comes from drawing the parallel to djb's Internet Mail 2000 [1], which tries to counter spams by changing mail storage to the sender's responsibility.

[1] https://cr.yp.to/im2000.html