1 files changed, 4 insertions, 0 deletions
diff --git a/lib/smtp.c b/lib/smtp.c
index 20dc85a5f..02ddaca0a 100644
--- a/lib/smtp.c
+++ b/lib/smtp.c
@@ -834,6 +834,10 @@ static CURLcode smtp_state_starttls_resp(struct Curl_easy *data,
   CURLcode result = CURLE_OK;
   (void)instate; /* no use for this yet */
 
+  /* Pipelining in response is forbidden. */
+  if(data->conn->proto.smtpc.pp.cache_size)
+    return CURLE_WEIRD_SERVER_REPLY;
+
   if(smtpcode != 220) {
     if(data->set.use_ssl != CURLUSESSL_TRY) {
       failf(data, "STARTTLS denied, code %d", smtpcode);