When you delete an account from the app, you also delete the key associated with it. In that way, if you decide later to log in, you can still use your username and password, but the keys will be missing. Мeanwhile, if you receive an encrypted email, it will be unreadable.
Usually, a deleted account in Secure Email will also be removed from the server after several days. Therefore, if you do not log in within the next few days, you will permanently lose it.