A personal letter will usually brighten up a person's day.

一封私人来信往往就能使人一天心情愉快。

《牛津词典》

A man who knows how to write a personal letter has a very powerful tool.

一个知道如何写私人信件的人有一个非常强大的工具。

youdao

This is a personal letter.

这是封私人邮件。

youdao

The pope made a highly unusual personal analysis of the case in a letter to Roman Catholic bishops around the globe that was made public Thursday by the Vatican.

VOA: standard.2009.03.12

U.S. President Barack Obama apologized to Afghan President Hamid Karzai in a personal letter.

WSJ: U.S. Pulls Out Advisers After Two Killed in Kabul

If you really want to move people, send them a personal letter or card, in your own hand writing.

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And then write a personal letter explaining any unusual circumstances--say, if you lost your job or reported a big, one-time capital gain.

FORBES: Magazine Article