3- Keep it simple  and with a layout people are used to.

Millions  use G Mail.
Why not  make a layout  not much different from  G Mail ?

This is not rocket science.

A TAB  for compose. 

A  tab  for mail Box , ( messages received and conversations ) .
 
 The messages not read could be in bold.


In the INBOX   tab  you might  have a list just of the messages   you received  in the last week only .

 A received message not yet read  could be in  bold like G Mail.

In the Inbox message  list it would be highlighted just  the first 4  letters of the mail  that send the message   , and the day and hour   conversation started.

From  :  Pedr......@gmail.com.
      Started : May 10. 10h 54 PM. ( USA eastern Time ).

In the server the wholle mail could be stored  . When people click to reply to the abbreviated mail..,the message would be send to the mail of the recipient.

Even if BN  profile got compromised  a user would not  acess to the real mails of the user.



No topics of the messages to keep things more private .

Favorite  mail tabs. I do not see  much need of that. After all the objective is privacy



2 tabs only :    COMPOSE AND INBOX.


At the most 3 : COMPOSE INBOX   CONTACTS.

(But  the full display  of contacts does defeat the very purpose of burn note I guess  )