diff --git a/src/Symfony/Component/Mime/Header/Headers.php b/src/Symfony/Component/Mime/Header/Headers.php
index c6e4c8eaaf..94614d9881 100644
--- a/src/Symfony/Component/Mime/Header/Headers.php
+++ b/src/Symfony/Component/Mime/Header/Headers.php
@@ -201,7 +201,7 @@ final class Headers
 
     public static function isUniqueHeader(string $name): bool
     {
-        return \in_array($name, self::UNIQUE_HEADERS, true);
+        return \in_array(strtolower($name), self::UNIQUE_HEADERS, true);
     }
 
     public function toString(): string
diff --git a/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php b/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
index e2eb75a697..89d8be01b0 100644
--- a/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
+++ b/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
@@ -212,6 +212,11 @@ class HeadersTest extends TestCase
         $this->assertFalse($headers->has('Message-ID'));
     }
 
+    public function testIsUniqueHeaderIsNotCaseSensitive()
+    {
+        $this->assertTrue(Headers::isUniqueHeader('From'));
+    }
+
     public function testToStringJoinsHeadersTogether()
     {
         $headers = new Headers();