笔友(bǐ yǒu) - pen friend, pen pal in Mandarin

笔友 (bǐyǒu) is a pen friend, literally "brush/pen" (笔, bǐ) plus "friend" (友, yǒu): a friend you know through writing rather than in person. It predates email, but the word has carried over happily to online pen-pal schemes and language exchanges.

It is a staple of GCSE-style writing tasks, since "describe your pen friend" is a classic prompt: 我的笔友住在中国 (wǒ de bǐyǒu zhù zài Zhōngguó), "my pen friend lives in China", is exactly the kind of sentence it shows up in.

Note that 笔友 (bǐyǒu) names the person, not the correspondence itself; the letters or messages you exchange are just 信 (xìn) or, for an online exchange, 邮件 (yóujiàn).