Accounts and Keys
Your Nostr key is your mailbox. There is no password and no account on a server: whoever holds the key holds the mail.
Creating an account
Create one in the app and it generates a key for you. The app shows a sync code, which is that key in a form you can paste into another device to get the same mailbox there.
Back up the sync code
It is the only way back into your mailbox. Nobody can reset it, and nobody can recover it for you. Keep it in a password manager, not in a screenshot.
Signing in
The sync code is what the login screen asks for. The other three are behind More options.
A signer keeps your key out of the app entirely: Nmail asks it to sign and to decrypt, and it answers. That is the setup to use if the same key also carries your social identity.
Several accounts
Add as many as you like and switch between them from the account menu. Each one keeps its own mailbox, its own settings and its own local data. Removing an account clears what is on the device and leaves the key untouched.
Relays
An account needs a relay list. Nmail asks for one before it lets you in, and keeps at least one relay in it. Those relays are where your mail arrives, so an account with none has no mailbox.
Deleting an account
Deleting asks the relays to forget you: a signed NIP-62 vanish request, aimed at
every relay this account could have reached rather than just your own. Relays
that honour it drop your events, the gift wraps addressed to you included, since
a wrap signed by a throwaway key is recognised by its p tag. The local data
goes with it. The key itself stays yours, and can be used again later.