邮局(yóu jú) - post office in Mandarin

邮局 (yóujú) is the post office. 邮 (yóu) means "post, mail" and turns up in a whole family of related words, and 局 (jú) means "office, bureau", the same character behind 警察局 (jǐngchájú), "police station".

Typical uses cluster around sending things: 去邮局寄信 (qù yóujú jì xìn), "go to the post office to send a letter", and 邮局寄包裹 (yóujú jì bāoguǒ), "send a parcel at the post office". A single post office is 一家邮局 (yì jiā yóujú), using the measure word for businesses and shops.

Do not mix it up with 邮箱 (yóuxiāng), which can mean either a postbox or, these days more often, an email inbox, nor with 邮件 (yóujiàn), "mail, post" as a general noun for the items themselves rather than the building.