关于使用SMTP编写邮件客户端的一些坑

问题来自于《自顶向下》套接字编程的第三题：https://github.com/moranzcw/Computer-Networking-A-Top-Down-Approach-NOTES/blob/master/SocketProgrammingAssignment/%E4%BD%9C%E4%B8%9A3-%E9%82%AE%E4%BB%B6%E5%AE%A2%E6%88%B7%E7%AB%AF/%E4%BD%9C%E4%B8%9A3-%E9%82%AE%E4%BB%B6%E5%AE%A2%E6%88%B7%E7%AB%AF-%E7%BF%BB%E8%AF%91.md

上手就是第一个坑。该书第二章以及该题的代码模板中皆没有提及授权登录的问题，实际上目前国内的email提供商（腾讯、163等）对于第三方登录都采取授权登录的方式（gmail因为一些众所周知的原因没试过），如果直接按照书中方式发送邮件会出现如下错误。

实际上我们可以将书上示例的smtp握手过程扩充成如下所示。

S: 220 xxx.com
C: HELO yourname
S: 250 greeting contents
C: AUTH LOGIN
S: 334 dXNlcm5hbWU6    #username
C: <your_username_in_base64>
S: 334 UGFzc3dvcmQ6    #password
C: <your_password_in_base64>
S: 235 AUTH successful
C: MAIL FROM: <from_mail_address>
S: 250 OK
C: RCPT TO: <to_mail_address>
S: 250 OK
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: main body of the message
             ...
             ...
C: .
S: 250 Mail accepted
C: QUIT
S: Bye

其中‘your_username_in_base64’和'your_password_in_base64'分别是你的邮箱用户名和授权码的base64编码，授权码可以从邮箱提供商获得，具体方法请百度。

看到这里，如果你和我一样迫不及待地去telnet一下smtp.163.com，尝试用上述的命令手工发送一封邮件的话，那你就会踩进第二个坑里。

好吧这个问题我花了两天，直到现在也没找出原因，实际上我后来尝试自己抓了一份outlook发送邮件的完整过程的包。

可以看到与我贴出的smtp握手过程基本一致，并且同样是输入我的邮箱地址的base64编码，得到的却是334肯定的回答，最终成功将邮件发出。初步猜测是windows键盘输入的问题。

这题最终的代码如下，这个smtp客户端能够正常工作，而不会像手动telnet那样报错。

from socket import *
msg = "\r\n I love computer networks!"
endmsg = "\r\n.\r\n"
# Choose a mail server (e.g. Google mail server) and call it mailserver 
mailserver = 'smtp.163.com'
serverPort = 25
# Create socket called clientSocket and establish a TCP connection with mailserver
fromaddress = 'decayfang@163.com'
toaddress = '1186565149@qq.com'

username = "ZGVjYXlmYW5nQDE2My5jb20="
password = "xxxxxxxxxxxxxxxxxxxxxxxx"

clientSocket = socket(AF_INET, SOCK_STREAM)
clientSocket.connect((mailserver, serverPort))

recv = clientSocket.recv(1024).decode()
print(recv)
if recv[:3] != '220':
    print('220 reply not received from server.')

# Send HELO command and print server response.
heloCommand = 'HELO Alice\r\n'
clientSocket.send(heloCommand.encode())
recv1 = clientSocket.recv(1024).decode()
print(recv1)
if recv1[:3] != '250':
    print('250 reply not received from server.')
    
authCommand = 'AUTH LOGIN\r\n'
clientSocket.send(authCommand.encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if recv[:3] != '334':
    print('334 reply not received from server.')
    
clientSocket.send((username + '\r\n').encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if recv[:3] != '334':
    print('334 reply not received from server.')
    
clientSocket.send((password + '\r\n').encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if recv[:3] != '235':
    print('235 reply not received from server.')

# Send MAIL FROM command and print server response.
# Fill in start
clientSocket.send(('MAIL FROM: <' + fromaddress + '>\r\n').encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if recv[:3] != '250':
    print('250 reply not received from server.')
# Fill in end

# Send RCPT TO command and print server response.
# Fill in start
clientSocket.send(('RCPT TO: <' + toaddress + '>\r\n').encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if recv[:3] != '250':
    print('250 reply not received from server.')
# Fill in end

# Send DATA command and print server response.
# Fill in start
clientSocket.send('DATA\r\n'.encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if recv[:3] != '354':
    print('354 reply not received from server.')
# Fill in end

# Send message data.
# Fill in start
message = 'From: ' + fromaddress + '\r\n'
message += 'To: ' + toaddress + '\r\n'
message += 'Subject: TESTING MAIL\r\n'
message += msg + '\r\n'
clientSocket.send(message.encode())
# Fill in end 

# Message ends with a single period.
# Fill in start
clientSocket.send(endmsg.encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if recv[:3] != '250':
    print('250 reply not received from server.')
# Fill in end

# Send QUIT command and get server response.
# Fill in start
clientSocket.send('QUIT'.encode())
# Fill in end

input('Press <Enter>')