Fixes #73 : allow Message to not has "To" header

src/Transport/Sendmail.php

@@ -148,8 +148,15 @@ protected function prepareRecipients(Mail\Message $message)
     {
         $headers = $message->getHeaders();
 
-        if (! $headers->has('to')) {
-            throw new Exception\RuntimeException('Invalid email; contains no "To" header');
+        $hasTo = $headers->has('to');
+        if (! $hasTo && ! $headers->has('cc') && ! $headers->has('bcc')) {
+            throw new Exception\RuntimeException(
+                'Invalid email; contains no at least one of "To", "Cc", and "Bcc" header'
+            );
+        }
+
+        if (! $hasTo) {
+            return '';
         }
 
         $to   = $headers->get('to');

test/Transport/SendmailTest.php

@@ -223,4 +223,37 @@ public function testTrimmedParameters()
 
         $this->assertSame('-R hdrs', $r->getValue($this->transport));
     }
+
+    public function testAllowMessageWithEmptyToHeaderButHasCcHeader()
+    {
+        $message = new Message();
+        $message->addCc('matthew@zend.com')
+                ->setSender('ralph.schindler@zend.com', 'Ralph Schindler')
+                ->setSubject('Testing Zend\Mail\Transport\Sendmail')
+                ->setBody('This is only a test.');
+
+        $this->transport->send($message);
+    }
+
+    public function testAllowMessageWithEmptyToHeaderButHasBccHeader()
+    {
+        $message = new Message();
+        $message->addBcc('zf-crteam@lists.zend.com', 'CR-Team, ZF Project')
+                ->setSender('ralph.schindler@zend.com', 'Ralph Schindler')
+                ->setSubject('Testing Zend\Mail\Transport\Sendmail')
+                ->setBody('This is only a test.');
+
+        $this->transport->send($message);
+    }
+
+    public function testDoNotAllowMessageWithoutToAndCcAndBccHeaders()
+    {
+        $message = new Message();
+        $message->setSender('ralph.schindler@zend.com', 'Ralph Schindler')
+                ->setSubject('Testing Zend\Mail\Transport\Sendmail')
+                ->setBody('This is only a test.');
+
+        $this->expectException(RuntimeException::class);
+        $this->transport->send($message);
+    }
 }