You'll get a dialogue box when exiting the program reminding you that there are still e-mails in your outbox.

当你退出程序时，你将会收到一个对话箱提醒你在发信信箱中还有一些邮件。

《柯林斯英汉双解大词典》

In Electronic Data Interchange (EDI), for example, there is no real notion of a service interface, since EDI systems usually have a single global inbox and outbox for messages.

例如，在电子数据交换(Electronic Data Interchange, EDI)中，不存在真正的服务接口概念，因为EDI系统通常有一个单个的全球的消息收件箱和发件箱。

youdao

One or more messages in your Outbox do not have a valid account specified.

您的发件箱中有一封或多封邮件没有指定有效的帐号。

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Los Angeles-based AEG expects to start selling its tickets through Outbox in the next six to 12 months, and to have nearly all its venues throughout the world on the new system within two years.

WSJ: Promoter Crowds Ticketmaster