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Asked: 2026-05-15T19:29:06+00:00

I am trying to use gmail’s SMTP server smtp.gmail.com to send mails using C

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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:a@gmail.com
530 5.7.0 Must issue a STARTTLS command first. g4sm73428740wae.2
STARTTLS
220 2.0.0 Ready to start TLS
MAIL FROM:a@gmail.com

and the connection is lost

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1 Answer

  1. Editorial Team

    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..

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