Would the sat "postage" go to the recipent or relay? I think it would also benefit from a bridge to legacy email with a mechanism to add the postage on inbound mail. Maybe LNURL in the header? Outbound messages could include return postage for real conversations that you want a reply to.
Discussion
To the relay .. the value is in holding the information. Clients would earn through value added service ( better display + more devices ) . Recipient gets the benefit of using the tool .
Bridging to the existing clients is an intermediary steps .. existing providers ( Gmail , Microsoft) would be forced to adapt if they want to keep their clout .. I guess nostr developers should least worry about this piece for one - it is hard to develop for all the email clients ..and second it would add to the mess ..and third the benefits wouldn't contrast clearly ..