snail mail
noun [ U ] informal humorous uk/ˈsneɪl ˌmeɪl/ us/ˈsneɪl ˌmeɪl/
平信,传统信件
letters or messages that are not sent by email, but in the post
We agreed the deal online, but we'll have to wait for snail mail to get the paperwork. 我们在网上达成了交易,但是还得等平信寄来书面文件。
Compare: electronic mail email noun