libcurl: can't authenticate through gmail with smtp and oauth2 - c++

Question

I'm trying to send an email through gmail using libcurl, smtp, and oauth2. I've successfully gotten an access_token and I've checked with
https://www.googleapis.com/oauth2/v1/tokeninfo?access_token={access_token}
to make sure that it is actually a valid access_token. However, whenever I try and authenticate and send the email, it always complains with an "Authentication Failure: 334". The code works perfectly when I replace the oauth2 line with a password line.
Here is the code:
static const char *payload_text[] = {
"Date: Mon, 29 Nov 2010 21:54:29 +1100\r\n",
"To: XXX#gmail.com\r\n",
"From: XXX#gmail.com\r\n",
"Subject: SMTP example message\r\n",
"\r\n", /* empty line to divide headers from body, see RFC5322 */
"The body of the message starts here.\r\n",
"\r\n",
"It could be a lot of lines, could be MIME encoded, whatever.\r\n",
"Check RFC5322.\r\n",
NULL
};
struct upload_status {
int lines_read;
};
static size_t payload_source(void *ptr, size_t size, size_t nmemb, void *userp) {
struct upload_status *upload_ctx = (struct upload_status *)userp;
const char *data;
if ((size == 0) || (nmemb == 0) || ((size*nmemb) < 1)) {
return 0;
}
data = payload_text[upload_ctx->lines_read];
if(data) {
size_t len = strlen(data);
memcpy(ptr, data, len);
upload_ctx->lines_read++;
return len;
}
return 0;
}
void send_email() {
CURL *curl;
CURLcode res = CURLE_OK;
struct curl_slist *recipients = NULL;
struct upload_status upload_ctx;
upload_ctx.lines_read = 0;
curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_USERNAME, "XXX#gmail.com");
curl_easy_setopt(curl, CURLOPT_XOAUTH2_BEARER, access_token.c_str());
curl_easy_setopt(curl, CURLOPT_URL, "smtp://smtp.gmail.com:587/");
curl_easy_setopt(curl, CURLOPT_USE_SSL, (long)CURLUSESSL_ALL);
curl_easy_setopt(curl, CURLOPT_MAIL_FROM, "XXX#gmail.com");
recipients = curl_slist_append(recipients, "XXXX#gmail.com");
curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);
curl_easy_setopt(curl, CURLOPT_READFUNCTION, payload_source);
curl_easy_setopt(curl, CURLOPT_READDATA, &upload_ctx);
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);
res = curl_easy_perform(curl);
if (res != CURLE_OK)
fprintf(stderr, "curl_easy_perform() failed: %s\n",
curl_easy_strerror(res));
curl_slist_free_all(recipients);
curl_easy_cleanup(curl);
}
}
and here is the latter half of the error message (where it differs and shows the failure):
< 250-mx.google.com at your service, [108.200.141.166]
< 250-SIZE 35882577
< 250-8BITMIME
< 250-AUTH LOGIN PLAIN XOAUTH XOAUTH2 PLAIN-CLIENTTOKEN
< 250-ENHANCEDSTATUSCODES
< 250 CHUNKING
> AUTH XOAUTH2
< 334
> dXNlcj1yY3dtY3Njcm9vZ2VAZ21haWwuY29tAWF1dGg9QmVhcmVyIHlhMjkuS2dCUVhCU1ZzSVpCQ1I4QUFBQzVucmdES2g3TzBQZ01JU25zRGlGSjc0c1M2N0Z5cUQycUJrVkgzdHFrV3cBAQ==
< 334 eyJzdGF0dXMiOiI0MDAiLCJzY2hlbWVzIjoiQmVhcmVyIiwic2NvcGUiOiJodHRwczovL21haWwuZ29vZ2xlLmNvbS8ifQ==
* Authentication failed: 334
* Closing connection 0
curl_easy_perform() failed: Login denied
Can anyone help explain what is going wrong here? Worse come to worse, I'll just end up using the username/password combination to log in, but I'd rather figure out what's messed up here (and I was under the impression that the oauth2 way was the preferred way to go).
Thanks in advance for any help.