diff --git a/smtp.php b/smtp.php
index 57b1901..aa5d7a9 100644
--- a/smtp.php
+++ b/smtp.php
@@ -173,6 +173,9 @@ class smtp
     $from = $this->cleanMail ($from);
     if (is_string ($to))
       $to = array ($to);
+    if (! is_array ($to) || count ($to) === 0)
+      throw new \Exception ("Can't send mail: no valid recipient provided",
+        500);
     $this->putLine ("MAIL FROM: $from\r\n");
     foreach ($to as $t)
     {