You set up an email server, let's say postfix, then have a nostr account for that server. The server must be able to speak normal email protocols like SMTP or POP3, and must also be able to speak the Nostr protocol. Emails must be encrypted (NIP-44) as they will also be considered Nostr notes and therefore duplicated among relays. The email server can encrypt them itself before converting them to notes. Then any nostr client supporting NIP-44 encrypted direct messages should display them normally. The nostr client can reply normally, but will probably require some special formatting to indicate who the intended recipient is. Could eventually make a NIP for this so clients could handle this issue transparently.

Then you have to set MX records to point your domain to the Nostr email server and so all the normal email server stuff.