diff --git a/src/PhpImap/Mailbox.php b/src/PhpImap/Mailbox.php
index edb065a0..bd8f22ae 100644
--- a/src/PhpImap/Mailbox.php
+++ b/src/PhpImap/Mailbox.php
@@ -2027,7 +2027,7 @@ protected function possiblyGetEmailAndNameFromRecipient(object $recipient): ?arr
             }
 
             if ('' !== \trim($recipientMailbox) && '' !== \trim($recipientHost)) {
-                $recipientEmail = \strtolower($recipientMailbox.'@'.$recipientHost);
+                $recipientEmail = \mb_strtolower($recipientMailbox.'@'.$recipientHost, 'UTF-8');
                 $recipientName = (\is_string($recipientPersonal) && '' !== \trim($recipientPersonal)) ? $this->decodeMimeStr($recipientPersonal) : null;
 
                 return [
@@ -2105,7 +2105,7 @@ protected function possiblyGetHostNameAndAddress(array $t): array
         }
 
         /** @var string */
-        $out[] = \strtolower($t[0]->mailbox.'@'.(string) $out[0]);
+        $out[] = \mb_strtolower($t[0]->mailbox.'@'.(string) $out[0], 'UTF-8');
 
         /** @var array{0:string|null, 1:string|null, 2:string} */
         return $out;
@@ -2142,7 +2142,7 @@ protected function searchMailboxFromWithOrWithoutDisablingServerEncoding(string
              * @return string
              */
             static function ($sender) use ($criteria): string {
-                return $criteria.' FROM '.\mb_strtolower($sender);
+                return $criteria.' FROM '.\mb_strtolower($sender, 'UTF-8');
             },
             $senders
         )));
diff --git a/tests/unit/Fixtures/Mailbox.php b/tests/unit/Fixtures/Mailbox.php
index 68c276b9..18ac0d09 100644
--- a/tests/unit/Fixtures/Mailbox.php
+++ b/tests/unit/Fixtures/Mailbox.php
@@ -13,6 +13,24 @@ public function decodeRFC2231ForTests(string $string): string
         return $this->decodeRFC2231($string);
     }
 
+    /**
+     * @return (null|string)[]|null
+     */
+    public function possiblyGetEmailAndNameFromRecipientForTests(object $recipient): ?array
+    {
+        return $this->possiblyGetEmailAndNameFromRecipient($recipient);
+    }
+
+    /**
+     * @param array<int, object> $t
+     *
+     * @return array{0:string|null, 1:string|null, 2:string}
+     */
+    public function possiblyGetHostNameAndAddressForTests(array $t): array
+    {
+        return $this->possiblyGetHostNameAndAddress($t);
+    }
+
     public function getImapPassword(): string
     {
         return $this->imapPassword;
diff --git a/tests/unit/MailboxAddressParsingTest.php b/tests/unit/MailboxAddressParsingTest.php
new file mode 100644
index 00000000..5165842c
--- /dev/null
+++ b/tests/unit/MailboxAddressParsingTest.php
@@ -0,0 +1,103 @@
+<?php
+/**
+ * Mailbox address parsing focused unit tests.
+ */
+declare(strict_types=1);
+
+namespace PhpImap;
+
+use PHPUnit\Framework\TestCase;
+use stdClass;
+
+final class MailboxAddressParsingTest extends TestCase
+{
+    public function testPossiblyGetEmailAndNameFromRecipientUsesMultibyteSafeLowercasing(): void
+    {
+        $recipient = new stdClass();
+        $recipient->mailbox = 'FÜR.MICH';
+        $recipient->host = 'EXAMPLE.DE';
+
+        $mailbox = new Fixtures\Mailbox('', '', '');
+
+        $this->assertSame(
+            ['für.mich@example.de', null],
+            $mailbox->possiblyGetEmailAndNameFromRecipientForTests($recipient)
+        );
+    }
+
+    public function testPossiblyGetHostNameAndAddressUsesMultibyteSafeLowercasing(): void
+    {
+        $sender = new stdClass();
+        $sender->mailbox = 'GRÜNE.POST';
+        $sender->host = 'EXAMPLE.DE';
+
+        $mailbox = new Fixtures\Mailbox('', '', '');
+
+        $this->assertSame(
+            ['EXAMPLE.DE', null, 'grüne.post@example.de'],
+            $mailbox->possiblyGetHostNameAndAddressForTests([$sender])
+        );
+    }
+
+    public function testSearchMailboxFromLowercasesSendersUsingUtf8RegardlessOfInternalEncoding(): void
+    {
+        $mailbox = new class('', '', '') extends Fixtures\Mailbox {
+            /** @var array{disableServerEncoding: bool, criteria: string[]} */
+            public array $capturedSearchMailboxArguments = [];
+
+            public function searchMailboxFromWithOrWithoutDisablingServerEncodingForTests(string $criteria, bool $disableServerEncoding, string $sender, string ...$senders): array
+            {
+                return $this->searchMailboxFromWithOrWithoutDisablingServerEncoding($criteria, $disableServerEncoding, $sender, ...$senders);
+            }
+
+            /**
+             * @param bool   $disableServerEncoding
+             * @param string $single_criteria
+             * @param string ...$criteria
+             *
+             * @return int[]
+             */
+            protected function searchMailboxMergeResultsWithOrWithoutDisablingServerEncoding($disableServerEncoding, $single_criteria, ...$criteria)
+            {
+                \array_unshift($criteria, $single_criteria);
+
+                $this->capturedSearchMailboxArguments = [
+                    'disableServerEncoding' => $disableServerEncoding,
+                    'criteria' => $criteria,
+                ];
+
+                return [];
+            }
+        };
+
+        $previousInternalEncoding = \mb_internal_encoding();
+
+        try {
+            \mb_internal_encoding('ISO-8859-1');
+
+            $this->assertSame(
+                [],
+                $mailbox->searchMailboxFromWithOrWithoutDisablingServerEncodingForTests(
+                    'ALL',
+                    true,
+                    'FÜR@EXAMPLE.DE',
+                    'für@example.de',
+                    'NOREPLY@EXAMPLE.DE'
+                )
+            );
+        } finally {
+            \mb_internal_encoding($previousInternalEncoding);
+        }
+
+        $this->assertSame(
+            [
+                'disableServerEncoding' => true,
+                'criteria' => [
+                    'ALL FROM für@example.de',
+                    'ALL FROM noreply@example.de',
+                ],
+            ],
+            $mailbox->capturedSearchMailboxArguments
+        );
+    }
+}