address book (pet names and metadata for global identifiers that is not shared globally) - nip81 may address this

folders - nip51 may address this but we need to define a set type for that purpose can include a, e, t tags i guess

search - while folders add organization, i still heavily use free text search to find stuff. meta based search is useful if i know roughly when but not enough specific words and phrases'

drafts - i sometimes start messages and dont send them until later, or use them as templates

distribution lists - i have multiple lists each including contacts and they may overlap..in nostr this needs to be explicit lists nip51 style, or implicit nip81 style

threading - ability to see history of a message exchange when replied to back and forth

master/detail with key meta - seeing from, date, subject for each email in a view and having a preview reading pane as i move through to show details

nice to haves. .....

bulk and scheduled sends - using a distro list and sending a message to each, all at once or scheduled intermittently over time or based upon the target's timezone

calendar integration

file attachments, extraction, scanning tools