Send mail through gmail SMTP server using Win API

Question

I am trying to use gmail's SMTP server smtp.gmail.com to send mails using C in Windows. I am able to connect to port 587 of the server, however the server responds by saying that STARTTLS/TLS is needed. Is there any Windows API call for starting a TLS connection ?

Should I even consider writing this application in C or use Python ?

Edit: Has anyone been able to send a mail by connecting to smtp.gmail.com using Telnet ? What I got was

220 mx.google.com ESMTP g4sm73428740wae.2
HELO hello
502 5.5.1 Unrecognized command. g4sm73428740wae.2
HELO hello.hello
250 mx.google.com at your service
MAIL FROM:[email protected]
530 5.7.0 Must issue a STARTTLS command first. g4sm73428740wae.2
STARTTLS
220 2.0.0 Ready to start TLS
MAIL FROM:[email protected]

and the connection is lost

Answer 1

You are going to need either a .NET wrapper around Microsoft's SmtpClient to create a DLL that can use the .NET features for SSL/TLS support through SMTP or use OpenSSL to handle the connections.

It would probably be beneficial for you to write this in C++. I am not familiar with Python, but I am sure there are libraries such as TLS Lite (Edit: see below, smtplib apparently provides you this functionality too).

Edit: Based on your edit, you have to have a program that knows how to handle the STARTTLS command. GMail requires secure connections and a username/password by default on all SMTP connections. You can connect to port 25, but you'll have to switch to the secure port after the initial connection. This is why, when you go to http://mail.google.com, it automatically switches to https connection. There was a whole big discussion about this on the Internet some time ago..

Answer 2

If you choose to write it in C then you will need a library like GNU SASL, which enables secure SMTP authentication. If you opt for C++ as was previously suggested then I would look at VMime, which is a free C++ library that uses GSASL. There is an example on the site that shows how to connect to GMail.

Answer 3

If you do not want to use OpenSSL or other third-party SSL/TLS library, then you can use Microsoft's own SSPI/SChannel APIs instead. It can be used on top of a socket, where the socket handles the low-level input/output of bytes, and you pass the bytes in/out of SSPI/SChannel functions for processing.