diff --git a/lib/smtp.c b/lib/smtp.c
index 81c069f7d..b73bb2d89 100644
--- a/lib/smtp.c
+++ b/lib/smtp.c
@@ -232,6 +232,11 @@ static int smtp_endofresp(struct pingpong *pp, int *resp)
   line += 4;
   len -= 4;
 
+  if(smtpc->state == SMTP_EHLO && len >= 4 && !memcmp(line, "SIZE", 4)) {
+    DEBUGF(infof(conn->data, "Server supports SIZE extension.\n"));
+    smtpc->size_supported = true;
+  }
+
   if(smtpc->state == SMTP_EHLO && len >= 5 && !memcmp(line, "AUTH ", 5)) {
     line += 5;
     len -= 5;
@@ -943,7 +948,7 @@ static CURLcode smtp_mail(struct connectdata *conn)
   }
 
   /* calculate the optional SIZE parameter */
-  if(conn->data->set.infilesize > 0) {
+  if(conn->proto.smtpc.size_supported && conn->data->set.infilesize > 0) {
     size = aprintf("%" FORMAT_OFF_T, data->set.infilesize);
 
     if(!size) {
diff --git a/lib/smtp.h b/lib/smtp.h
index 38fd1b70a..d68a659b5 100644
--- a/lib/smtp.h
+++ b/lib/smtp.h
@@ -66,6 +66,8 @@ struct smtp_conn {
   struct curl_slist *rcpt; /* Recipient list */
   bool ssldone;            /* Is connect() over SSL done? only relevant in
                               multi mode */
+  bool size_supported;     /* If server supports SIZE extension according to
+                              RFC 1870 */
 };
 
 extern const struct Curl_handler Curl_handler_smtp;