1.
mailbox, postbox
A public box for depositing outgoing mail.
ポスト is from English "post" and has two main meanings in Japanese.
Sense 1 - mailbox
Public red boxes where you deposit outgoing mail.
Sense 2 - POSITION/POST
A job position, especially in business or organizations.
Cultural note
Japanese mailboxes are typically bright red (赤いポスト), making them easy to spot. There are two slots: 手紙・はがき (letters/postcards) and その他 (other mail/large items).