Here is another excellent Adobe XD Kit for your mailbox design project.
This template has a designed inbox with received emails and a single email view structure with attachments.
There is an input for the reply below email text, and on the left side of the mailbox, you have nice navigation with inbox, flagged, trash, draft, sent, and more cards. All cards feature simple icons.
There are also category folders with different colors, and below that is a compose button.
This freebie was created by Neelesh Chaudhary and you can download it 100% free.