开发信的英文：
Promotional Letter

promotional是什么意思：
adj. 促销的；增进的；奖励的

And the show will help promote it.
而这次展览会有利于促销它。

He is to promote by a presidential order.
他即将按主管策令升职。

He was promoted to manager.
他被提升为经理。

To promote a concert is not easy.
筹办一场音乐会并不容易。

Exchange promotes development.
交流促进发展。

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。