You call it an "empty envelope" to emphasize that it should eventually contain a "letter" (perhaps a business letter) before you can send it by "post".

称其为“空envelope”是为了强调在通过“投递”发出它之前，它最终应该包含一封“信”(也许是商业信)。

youdao

One afternoon, he enters the post office two minutes before closing. The lobby is empty but for the postman and his broom.

一天下午，他在邮局关门前两分钟赶了进去。大厅里除了邮差和他的扫帚以外别无他人。

youdao

This post is too important, so in the past year, the whole seven south where he Ephraim hex mansion always purple as a senior is always empty.

这个职务过于重要，所以或过去地整整七年之中，南玥所或地紫莲妖术府总学长一职一直都似空着嗄。

youdao

This post is too important, so in the past year, the whole seven south where he Ephraim hex mansion always purple as a senior is always empty.

这个职务过于重要，所以或过去地整整七年之中，南玥所或地紫莲妖术府总学长一职一直都似空着嗄。

youdao