a simple sniffer,why does not this simple program work well?

[sunwukong]

These time I am reading a wondeful book,it's name is "GRAY HAT PYTHON".And in the 4 chapter,it make a mail sniffer,but in my computer,it can not sniffer the mail login data packets.
it's code:

#coding=utf-8
from scapy.all import *
def packet_callback(packet):
    if packet[TCP].payload:
        mail_packet = str(packet[TCP].payload)
        if "user" in mail_packet.lower() or "pass" in mail_packet.lower():
#do not mate the if code
            print ("[*] Server: %s" % packet[IP].dst)
            print ("[*] %s" % packet[TCP].payload)
sniff(filter="tcp port 110 or tcp port 25 or tcp port 143", prn=packet_callback, store=0)
 

pop3.163.com is a chinese mail server,I telnet it use port 110,and try "user and pass",but it is not sniffed by my program.
Is it use SSL or other skill to encrypt the packet?
should any kind people help me?
Thanks in advance!