I was reading this post having regard to data size from the beginning. From the technology point of view data size doesn't matter blockchain protocols can handle it, but the real issue is the price of that storage.

If it comes to fees 0.001 or even 0.01 dollar is fair enough I think. Personally I'll pay that fee for the privacy and ad-free mailbox. What's more this fee will significantly reduce the numer of spam and marketing e-mails.

Unfortunately there is a long, long way in order to mass adoption of blockchain based mailboxes because of services like GMail, Yahoo etc...
First off all, there is need for the integration between traditional and Blockchain e-mail services and then we are on straight way to disrupt this area.