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 ..