diff --git a/src/Sender/MailToFileSender.php b/src/Sender/MailToFileSender.php
index 536347b..b05d93c 100644
--- a/src/Sender/MailToFileSender.php
+++ b/src/Sender/MailToFileSender.php
@@ -47,15 +47,17 @@ public function send(iterable $frames): void
     /**
      * Get normalized email address for file or directory name.
      *
-     * @return non-empty-string
+     * @return non-empty-string|null
      */
-    private static function normalizeEmail(string $email): string
+    private static function normalizeEmail(?string $email): ?string
     {
-        return \preg_replace(
+        $normalized = \preg_replace(
             ['/[^a-z0-9.\\- @]/i', '/\s+/'],
             ['!', '_'],
-            $email,
+            (string) $email,
         );
+
+        return $normalized === '' ? null : $normalized;
     }
 
     /**
@@ -63,14 +65,14 @@ private static function normalizeEmail(string $email): string
      */
     private static function fetchDirectories(Message\Smtp $message): array
     {
-        return
-            \array_filter(
-                \array_unique(
-                    \array_map(
-                        static fn(Contact $c) => self::normalizeEmail($c->email),
-                        \array_merge($message->getBcc(), $message->getTo()),
-                    ),
+        return \array_values(\array_filter(
+            \array_unique(
+                \array_map(
+                    static fn(Contact $c): ?string => self::normalizeEmail($c->email),
+                    \array_merge($message->getBcc(), $message->getTo()),
                 ),
-            );
+            ),
+            static fn(?string $dir): bool => $dir !== null,
+        ));
     }
 }
diff --git a/src/Traffic/Message/Smtp.php b/src/Traffic/Message/Smtp.php
index 0cd1659..b6ebf3e 100644
--- a/src/Traffic/Message/Smtp.php
+++ b/src/Traffic/Message/Smtp.php
@@ -133,7 +133,7 @@ public function getSender(): array
     {
         $addrs = \array_unique(\array_merge((array) ($this->protocol['FROM'] ?? []), $this->getHeader('From')));
 
-        return \array_map([$this, 'parseContact'], $addrs);
+        return \array_map(self::parseContact(...), $addrs);
     }
 
     /**
@@ -141,7 +141,7 @@ public function getSender(): array
      */
     public function getTo(): array
     {
-        return $this->normalizeAddressList($this->getHeader('To'));
+        return self::normalizeAddressList($this->getHeader('To'));
     }
 
     /**
@@ -149,7 +149,7 @@ public function getTo(): array
      */
     public function getCc(): array
     {
-        return $this->normalizeAddressList($this->getHeader('Cc'));
+        return self::normalizeAddressList($this->getHeader('Cc'));
     }
 
     /**
@@ -160,7 +160,7 @@ public function getCc(): array
      */
     public function getBcc(): array
     {
-        return $this->normalizeAddressList($this->protocol['BCC'] ?? []);
+        return self::normalizeAddressList($this->protocol['BCC'] ?? []);
     }
 
     /**
@@ -168,7 +168,7 @@ public function getBcc(): array
      */
     public function getReplyTo(): array
     {
-        return $this->normalizeAddressList($this->getHeader('Reply-To'));
+        return self::normalizeAddressList($this->getHeader('Reply-To'));
     }
 
     public function getSubject(): string
@@ -187,7 +187,7 @@ public function getMessage(MessageFormat $type): ?Field
         return null;
     }
 
-    private function parseContact(string $line): Contact
+    private static function parseContact(string $line): Contact
     {
         if (\preg_match('/^\s*+(?<name>.*?)\s*<(?<email>.*)>\s*$/', $line, $matches) === 1) {
             $name = match (true) {
@@ -206,26 +206,27 @@ private function parseContact(string $line): Contact
     }
 
     /**
-     * @return array<Contact>
+     * @return list<Contact>
      */
-    private function parseDestinationAddress(string $line): array
+    private static function parseDestinationAddress(string $line): array
     {
         // if this is a group recipient
         if (\preg_match('/^[^"]+:(.*);$/', $line, $matches) === 1) {
             $line = $matches[1];
         }
 
-        $emailList = \array_map('trim', \explode(',', $line));
-        return \array_map([$this, 'parseContact'], $emailList);
+        $emailList = \array_map(\trim(...), \explode(',', $line));
+        return \array_map(self::parseContact(...), $emailList);
     }
 
     /**
-     * @return array<Contact>
+     * @param list<string> $param
+     * @return list<Contact>
      */
-    private function normalizeAddressList(array $param): array
+    private static function normalizeAddressList(array $param): array
     {
         return \array_merge(
-            ...\array_map([$this, 'parseDestinationAddress'], $param),
+            ...\array_map(self::parseDestinationAddress(...), $param),
         );
     }
 }