密件的英文：
confidential letter

参考例句：

Three outward-bound envelopes, all marked Private and Confidential, had come through the mailroom
三封标明“密件亲启”的外寄信件送到了邮件收发室。

Mail BCC
邮件密件抄送

BCC self
密件抄送(BCC)给自己

Show the Blind-Carbon-Copy field
显示"密件抄送"字段

BCC Other
密件抄送给别人

Declare (information,documents,etc)to be officially secret and available only to certain people
将(资料、文件等)定为官方密件而仅供某些人使用.

confidential是什么意思：
adj. 秘密的；信任或亲密的；担任机密工作的

confidentiality of contracts
合同的保密性

He has been on confidential missions to Berlin.
他前往柏林执行秘密任务。

He told me the confidential information.
他向我透露了这个秘密的信息。

Don't be too confidential with strangers.
不要过份相信陌生人。

She is a confidential secretary of Mr. Smith.
她是史密斯先生的一个机要秘书。

letter是什么意思：
n. 字母；信；字面意义；证书，许可证；文学
v. 用字母标明，写字母于；写印刷体字母；赢得校名缩写字母标志

in letter and in spirit
在字面意义和精神实质上|无论形式和实际

The letter was a forgery.
这封信是伪造的。

Post the letter through the slit in the letter box.
把这封信从信箱的投信口投进去。

Imprint a postmark on a letter =imprint a letter with a postmark
盖邮戳于信上

The letter is directed to him.
这封信是写给他的。