The Nostr mail would be so damn awesome if you could set a minimum sat threshold to recieve messages from people that are not in your contact list. An email contact list would need to be encrypted and not the same as the note1-follow list because privacy, and should contain other metadata like nip-05, nickname, realname, other private data, but ultimately the client can be where the contact list is unlocked and a payment threshold enforced. The only thing email has over nostr DM is the ability to create one-off distribution lists (To, Cc, Bcc), and attachments included in the message itself.
Distribution lists can be done akin to private message groups, and encrypted files can be done the same way as images on nostr, but the existing note1 oriented app is not suitable for email and likely would need a separate client based on an email client (yes I still hate email, but normies expect this mess).
Could email context sorting be solved differently than LDA sieves and IMAP folders? I'd like to know. 

 Also, ecash payments could be supported by the nostr email client.