+/* https://developers.google.com/gmail/xoauth2_protocol */
+static int
+sm_auth_xoauth2(const char *client_res)
+{
+ int status = smtalk(SM_AUTH, "AUTH XOAUTH2 %s", client_res);
+ if (status == 235) {
+ /* It worked! */
+ return RP_OK;
+ }
+
+ /*
+ * Status is 334 and sm_reply.text contains base64-encoded JSON. As far as
+ * epg can tell, no matter the error, the JSON is always the same:
+ * {"status":"400","schemes":"Bearer","scope":"https://mail.google.com/"}
+ * I tried these errors:
+ * - garbage token
+ * - expired token
+ * - wrong scope
+ * - wrong username
+ */
+ /* Then we're supposed to send an empty response ("\r\n"). */
+ smtalk(SM_AUTH, "");
+ /*
+ * And now we always get this, again, no matter the error:
+ * 535-5.7.8 Username and Password not accepted. Learn more at
+ * 535 5.7.8 http://support.google.com/mail/bin/answer.py?answer=14257
+ */
+ return RP_BHST;
+}
+