目的是通过关键字查找到所有邮件并下载附件，程序跑起来没问题。但是运行到一半的时候就终止并报以下错误。请朋友帮忙看看，可能是哪里出的错。十分感谢！
Traceback (most recent call last):
File "C:\Users\lenovo\Desktop\测试\pop3receiveemail.py", line 88, in
resp, lines, octets = email_server.retr(i)
File "C:\Users\lenovo\Desktop\测试\lib\poplib.py", line 248, in retr
return self._longcmd('RETR %s' % which)
File "C:\Users\lenovo\Desktop\测试\lib\poplib.py", line 183, in _longcmd
return self._getlongresp()
File "C:\Users\lenovo\Desktop\测试\lib\poplib.py", line 159, in _getlongresp
resp = self._getresp()
File "C:\Users\lenovo\Desktop\测试\lib\poplib.py", line 152, in _getresp
raise error_proto(resp)
poplib.error_proto: b' '
