only when -f option is set, set -f option automatically

src/Transport/Sendmail.php

@@ -259,6 +259,9 @@ protected function prepareParameters(Mail\Message $message)
         }
 
         $parameters = (string) $this->parameters;
+        if (preg_match('/^\-f.+$/', $parameters) || strpos($parameters, ' -f') !== false) {
+            return $parameters;
+        }
 
         $sender = $message->getSender();
         if ($sender instanceof AddressInterface) {

test/Transport/SendmailTest.php

@@ -27,6 +27,7 @@ class SendmailTest extends TestCase
     public $message;
     public $additional_headers;
     public $additional_parameters;
+    public $operating_system;
 
     public function setUp()
     {
@@ -252,4 +253,30 @@ public function testDoNotAllowMessageWithoutToAndCcAndBccHeaders()
         $this->expectException(RuntimeException::class);
         $this->transport->send($message);
     }
+
+    public function testNotSetOptionAutomaticallyOnLeadingF()
+    {
+        if ($this->operating_system == 'WIN') {
+            $this->markTestSkipped('This test is *nix-specific');
+        }
+
+        $message = $this->getMessage();
+        $this->transport->setParameters('-f\'[email protected]\'');
+
+        $this->transport->send($message);
+        $this->assertEquals('-f\'[email protected]\'', $this->additional_parameters);
+    }
+
+    public function testNotSetOptionAutomaticallyOnMiddleF()
+    {
+        if ($this->operating_system == 'WIN') {
+            $this->markTestSkipped('This test is *nix-specific');
+        }
+
+        $message = $this->getMessage();
+        $this->transport->setParameters('-bs -f\'[email protected]\'');
+
+        $this->transport->send($message);
+        $this->assertEquals('-bs -f\'[email protected]\'', $this->additional_parameters);
+    }
 }