diff --git a/Header/AbstractHeader.php b/Header/AbstractHeader.php
index 9994ec6..3dc7faf 100644
--- a/Header/AbstractHeader.php
+++ b/Header/AbstractHeader.php
@@ -188,6 +188,20 @@ protected function getEncodableWordTokens(string $string): array
             $tokens[] = $encodedToken;
         }
 
+        foreach ($tokens as $i => $token) {
+            // whitespace(s) between 2 encoded tokens
+            if (
+                0 < $i
+                && isset($tokens[$i + 1])
+                && preg_match('~^[\t ]+$~', $token)
+                && $this->tokenNeedsEncoding($tokens[$i - 1])
+                && $this->tokenNeedsEncoding($tokens[$i + 1])
+            ) {
+                $tokens[$i - 1] .= $token.$tokens[$i + 1];
+                array_splice($tokens, $i, 2);
+            }
+        }
+
         return $tokens;
     }
 
diff --git a/Tests/Header/MailboxHeaderTest.php b/Tests/Header/MailboxHeaderTest.php
index 2fc8e1e..bddadee 100644
--- a/Tests/Header/MailboxHeaderTest.php
+++ b/Tests/Header/MailboxHeaderTest.php
@@ -62,6 +62,14 @@ public function testUtf8CharsInLocalPart()
     {
         $header = new MailboxHeader('Sender', new Address('fabïen@symfony.com'));
         $this->assertSame('fabïen@symfony.com', $header->getBodyAsString());
+
+        // name with single space
+        $header = new MailboxHeader('Sender', new Address('fabïen@symfony.com', 'Fabïen Pötencier'));
+        $this->assertSame('=?utf-8?Q?Fab=C3=AFen_P=C3=B6tencier?= <fabïen@symfony.com>', $header->getBodyAsString());
+
+        // name with double spaces
+        $header = new MailboxHeader('Sender', new Address('fabïen@symfony.com', 'Fabïen  Pötencier'));
+        $this->assertSame('=?utf-8?Q?Fab=C3=AFen__P=C3=B6tencier?= <fabïen@symfony.com>', $header->getBodyAsString());
     }
 
     public function testToString()