An SAE is an envelope on which you have already written your own name and address, and usually already put a stamp, so that someone else can post something back to you without extra work or extra cost. The letters stand for stamped addressed envelope.

An SAE is an envelopeSAE是一種信封 on which you have already written your own name and address上面已經寫好自己的姓名地址 and usually already put a stamp通常也已經貼好郵票 so that someone else can post something back to you好讓對方可以把東西寄回給你 without extra work or extra cost不需要額外花力氣或花錢。The letters stand for這幾個字母代表 stamped addressed envelopestamped addressed envelope（已貼郵票、寫好地址的信封）。

This is mainly British English, and it belongs to letters and forms more than to chat: “Please enclose an SAE.” Though actually, younger people, and people in the United States, more often say SASE (self-addressed stamped envelope), or they use email and never need the envelope at all. The idea is the same: you have already written where the reply should go, and you have usually already paid for the stamp.

This is mainly British English這主要是英式英文的說法, and it belongs to letters and forms more than to chat比較常出現在書信和表格裡而不是聊天中: "Please enclose an SAE."「請附上一個SAE。」 Though actually不過, younger people, and people in the United States年輕人和美國人 more often say SASE (self-addressed stamped envelope)比較常說SASE（self-addressed stamped envelope）, or they use email and never need the envelope at all或者直接用email，根本不需要信封。The idea is the same概念是一樣的: you have already written where the reply should go你已經先寫好回信要寄去哪裡, and you have usually already paid for the stamp而且通常也已經先付了郵資。

You say it as three letters (S-A-E), not as a new English word.

You say it as three letters (S-A-E)你要把它唸成三個字母（S-A-E）, not as a new English word而不是當作一個新的英文單字來唸。

It is a practical, slightly old-fashioned bit of office and postal English.

It is a practical, slightly old-fashioned bit這是一個實用、又帶點老派味道的說法, of office and postal English屬於辦公室和郵務用語。