I’m working on a C program in Windows that needs to send emails through Gmail’s SMTP service. I can successfully establish a connection to port 587 on smtp.gmail.com, but I’m running into an issue where the server requires TLS encryption before accepting any mail commands.
When I try to send emails, the server responds with an error message stating that STARTTLS is required. I’ve been searching for Windows API functions that can handle TLS connections but haven’t found a clear solution yet.
I’m wondering if there are specific Windows API calls I should be using to establish the TLS handshake? Or would it be more practical to switch to a different programming language like Python for this task?
Has anyone successfully managed to send emails through Gmail’s SMTP server using low-level socket programming in C on Windows? Any guidance on handling the TLS requirement would be really helpful.
Raw Winsock and TLS with Gmail is a nightmare. Gmail’s security requirements are brutal, and you’ll end up writing tons of boilerplate code just for certificate validation and TLS negotiation. Windows SSPI functions work, but they’re a pain to implement right. The certificate chain validation really got me - Gmail’s certificates change and you need solid error handling for that. I spent weeks debugging auth issues and TLS handshake failures before I gave up and switched to libcurl, which handles everything internally. If you’re dead set on pure C and Windows APIs, nail the STARTTLS command sequence and prepare to debug security context initialization forever.
I encountered a similar challenge while developing a notification system with C on Windows. You can achieve TLS support using the Windows API, but it involves a considerable amount of effort. Specifically, you would utilize the Schannel API, which includes functions such as AcquireCredentialsHandle, InitializeSecurityContext, and EncryptMessage/DecryptMessage. Properly managing the TLS handshake after sending the STARTTLS command can be complex. If you’re considering implementing OAuth2 for Gmail, be prepared for additional complications. I personally faced numerous issues related to certificate handling that could have been circumvented by using a more robust library. While it is feasible to send emails in C utilizing the Windows API, I would strongly recommend exploring alternatives like libcurl with SSL or OpenSSL, as they simplify the process significantly.
Just use OpenSSL with MinGW or Visual Studio. Building Gmail SMTP with pure WinAPI is reinventing the wheel - badly. I’ve watched people burn months on SChannel integration when OpenSSL handles it in 20 lines. Gmail’s OAuth requirements will make you regret not switching anyway.