It just dawned on me that a large email provider like Yahoo, gMail or Outlook could save lots of storage by just having a pointer to the email in the Sent and InBox, so if both are in the same email provider then you only store the message once, further if it has an attachment it could be lots of space savings. But then it gives further thought as to if the 3 major email provider federated operations, then one of them could pull the email out from the sending provider with a pointer, rather than storing it.
If you evolve that concept further, if the sender then deletes the email it would also disappear from the recipients inbox, which would be great for those drunk emails one sends, where you wish you hadn't sent it. Or it could also operate in a way that that can't happen on deletion copy it to the recipient provider. Of course if you POP or IMAP then I guess you still have a copy either way.
I have been thinking lately about some of the systems we have built over the years with open source components that mostly dictate the behavior of the whole system and you can't innovate as much. Maybe if we wrote more things from scratch we could do interesting things.