TypeError：无法连接电子邮件中的“ str”和“ list”对象

我正在使用python发送电子邮件。现在，我想通过电子邮件从列表中发送条目，但是遇到一个错误，提示“ TypeError：无法连接'str'和'list'对象”，我不知道要对其进行调试。以下是我的代码。我还是这个语言的新手（3周），所以有点背景知识。

import smtplib
x = [2, 3, 4] #list that I want to send
to = '' #Recipient
user_name = '' #Sender username
user_pwrd = '' #Sender Password
smtpserver = smtplib.SMTP("mail.sample.com",port)
smtpserver.ehlo()
smtpserver.starttls()
smtpserver.ehlo()
smtpserver.login(user_name,user_pwrd)

#Header Part of the Email       
header = 'To: '+to+'\n'+'From: '+user_name+'\n'+'Subject: \n'
print header

#Msg 
msg = header + x #THIS IS THE PART THAT I WANT TO INSERT THE LIST THAT I WANT TO SEND. the type error occurs in this line

#Send Email     
smtpserver.sendmail(user_name, to, msg)
print 'done!'

#Close Email Connection
smtpserver.close()