SMTP 协议是push协议，因此不能用于收邮件。POP3 协议是 Post Office Protocol。
$ telnet pop.qq.com 110
Trying 59.37.97.57...
Connected to pop.qq.com.
Escape character is '^]'.
+OK QQMail POP3 Server v1.0 Service Ready(QQMail v2.0)
user 1234567890
+OK
pass xxxxxxx
+OK
list
1 119
2 22
retr 1
+OK
(data bala bala bala)
.
dele 2
+OK