noun
outbox
Used in email clients and messaging apps for the folder where outgoing messages are held before sending.
メールを送信すると、まず送信箱に入ります。
When you send an email, it first goes into the outbox.
受信箱 is the inbox, where incoming messages are received, while 送信箱 is the outbox for outgoing messages.
Compound of 送信 (transmission) and 箱 (box), directly calqued from English 'outbox'.