Number + 封 + 信/邮件
封 (fēng) is a classifier specifically used for letters, envelopes, and sealed documents. When counting letters or written correspondence, 封 is the appropriate measure word to use.
我昨天收到了三封信。
Wǒ zuótiān shōudào le sān fēng xìn.
I received three letters yesterday.
她给远方的朋友寄了一封信。
Tā gěi yuǎnfāng de péngyou jì le yì fēng xìn.
She sent a letter to a friend far away.
桌上放着两封还没拆开的信。
Zhuō shàng fàng zhe liǎng fēng hái méi chāikāi de xìn.
There are two unopened letters on the desk.