Connect to SMTP (SSL or TLS) using Python

Question

I am attempting to connect to the Gmail SMTP mail server and perform tasks as outlined by the skeleton code given to me. Only the use of sockets is allowed (so not the smtplib). I need to: send HELO command, MAIL FROM, RCPT TO, and DATA.

There are many cases of similar problems posted, but they haven't received the proper answer. For example: Implementing Transport Layer Security in Python - Simple Mail Client

The program is required to connect to smtp.gmail.com over port 587. I've taken two different approaches:

1. Using STARTTLS:

   mailserver = 'smtp.gmail.com'
   clientSocket = socket(AF_INET, SOCK_STREAM)
   clientSocket.connect((mailserver, 587))
   recv = clientSocket.recv(1024)
   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)
   recv1 = clientSocket.recv(1024)
   print recv1
   if recv1[:3] != '250':
       print '250 reply not received from server.'
   
   #Send MAIL FROM command and print server response.
   command = "STARTTLS\r\n"
   clientSocket.send(command)
   recvdiscard = clientSocket.recv(1024)
   print recvdiscard
   clientSocket.send("MAIL From: email\r\n")
   recv2 = clientSocket.recv(1024)
   print recv2
   if recv2[:3] != '250':
       print '250 reply not received from server.'
   

2. Using SSL:

   clientSocketSSL = ssl.wrap_socket(clientSocket)
   

   Then clientSocketSSL replaces all instances of clientSocket. The STARTTLS lines are also removed and import ssl is added to the top.

When using the first method, the MAIL FROM: command isn't returning anything. I'm getting the following output:

250 mx.google.com at your service

220 2.0.0 Ready to start TLS

250 reply not received from server.

When using SSL, I'm getting the same as the linked post:

ssl.SSLError: [Errno 1] _ssl.c:504: error:140770FC:SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol

Am I missing something here? I guess my best option is to use TLS but I have no idea how to go about that... is there something wrong with my MAIL FROM command?

Answer 1

When using SSL, you need to connect to port 465 instead of port 587. If you use STARTTLS, you still need to use ssl.wrap_socket, you just do it later - specifically, after receiving the 220 response to the STARTTLS command. After doing STARTTLS, you're supposed to do HELO again, since the server is supposed to forget anything that happened before the STARTTLS.

In either case, the servers at smtp.google.com ports 465 and 587 still won't return a 250 response to the MAIL command, since they require that you are authenticated before you send mail. You'll get a 530 response instead. You'll need to use the AUTH command with your gmail.com credentials to authenticate before you can use MAIL successfully on those servers.

If you don't want to authenticate, and depending on the details of what you need to do, you could try using port 25 of the server found in gmail.com's MX record. At the moment, the server is gmail-smtp-in.l.google.com and supports STARTTLS.

Answer 2

After STARTTLS, call

clientSocket = ssl.wrap_socket(clientSocket)