Snail¹ mail refers to a letter or package physically² delivered by a postal³ service, suggesting that the service is as slow as a snail. People would like to use this term especially when the object mailed could have been sent electronically via e-mail and arrived at the destination instantly.

“蜗牛邮件”指通过人工邮政投递的信件或包裹，snail在这里暗示这种投递方式的速度跟蜗牛一样慢。当某件邮品本可以通过电子邮件即刻送达收件人却没有这样做的时候，人们就用会snail mail来表达两种投递方式的区别。

For example:

Paranoid about "hackers⁴" spying on him, my boss insists on sending all correspondence via snail mail, which means I can't use my spam filter to ignore him.

因为老担心有黑客监视，我老板坚持所有的函件用邮政投递，这就意味着我没法用垃圾邮件过滤器把他的邮件给过滤掉了。