I think if we throw away the direct similarity between the blockchain as a transaction log vs a spoolchain as a massive collective megaspool and instead go with the concept of a spoolchain being a log of pointers to storage bins which are the actual distributed storage for a destination group, we get rid of most of the problems with storage size.
Eg. I am firstname.lastname@example.org, I want a storage bin for this account. A storage bin could then be either someone who just has email@example.com (say on a massive scale, the u shard for the provider.com domain) or someone who has the entire storage bin for provider.com, or someone who has the entire storage bin for .com
Couple this with open access to the storage bins from any and all (the more the better, the heavier replicated they are the easier / faster end users have access to their mail) and an economic incentive to participate in storage propagation (mailcoin? n mailcoins per n mb of storage, charge mailcoins for access to the storage bin, whatever model makes the most economic sense) and we might be onto something. It also handles deletion and allows users to "un-send" mail by deleting the mail from the storage bin it exists within. The blockchain in bitcoin does not allow for transaction deletion operations, but that does not by any means rule out the possibility of deleting the position in a storage bin where the spoolchain in mailcoin points to.