SMTP TLS client with SChannel

I am working on developing an SMTP client to connect with Gmail via TLS 1.x, but I am getting an error from AcquireCredentialsHandle() and displaying an error like "Error 0x80090308 returned by InitializeSecurityContext":

Status = g_pSSPI->AcquireCredentialsHandle(
    NULL,                       // Name of principal
    (SEC_WCHAR*) UNISP_NAME_A,  // Name of package
    SECPKG_CRED_OUTBOUND,       // Flags indicating use
    NULL,                       // Pointer to logon ID
    &SchannelCred,              // Package specific data
    NULL,                       // Pointer to GetKey() func
    NULL,                       // Value to pass to GetKey()
    phCreds,                    // (out) Cred Handle
    &tsExpiry );                // (out) Lifetime (optional)

if(Status != SEC_E_OK)
    printf("**** Error 0x%x returned by AcquireCredentialsHandle\n", Status);

Can anyone provide me a working SMTP client with TLS by using SChannel? I have tried with a sample available on the Internet, but unfortunately it didn't work.