diff --git a/ext/standard/VmMail.php b/ext/standard/VmMail.php
index f0e00a91a8e..a6963e79a55 100644
--- a/ext/standard/VmMail.php
+++ b/ext/standard/VmMail.php
@@ -5,10 +5,13 @@
 namespace PHPCompiler\ext\standard;
 
 use PHPCompiler\Frame;
+use PHPCompiler\VM\EnumCaseSupport;
 use PHPCompiler\VM\ErrorReporter;
+use PHPCompiler\VM\HashTable;
+use PHPCompiler\VM\Variable;
 
 /**
- * mail() sendmail transport — php-src ext/standard/mail.c `php_mail()` (#3285).
+ * mail() sendmail transport — php-src ext/standard/mail.c `php_mail()` (#3285, #21432).
  *
  * PHP-in-PHP: popen(sendmail_path) + RFC822 envelope on stdin; no new runtime C.
  */
@@ -20,6 +23,229 @@ final class VmMail
     /** sysexits.h EX_TEMPFAIL — php_mail() also treats as success. */
     private const EX_TEMPFAIL = 75;
 
+    /** Headers that reject array values (php_mail_build_headers PHP_MAIL_BUILD_HEADER_CHECK). */
+    private const STRING_ONLY_HEADERS = [
+        'orig-date' => true,
+        'from' => true,
+        'sender' => true,
+        'reply-to' => true,
+        'cc' => true,
+        'bcc' => true,
+        'message-id' => true,
+        'references' => true,
+        'in-reply-to' => true,
+    ];
+
+    /**
+     * Coerce mail() $additional_headers — string or array (php-src mail.c; #21432).
+     *
+     * @throws \TypeError
+     * @throws \ValueError
+     */
+    public static function coerceAdditionalHeaders(Variable $arg): ?string
+    {
+        $arg = $arg->resolveIndirect();
+        if (Variable::TYPE_NULL === $arg->type) {
+            return null;
+        }
+        if (Variable::TYPE_STRING === $arg->type) {
+            $headers = $arg->toString();
+            VmString::rejectNullByteBuiltinStringArg($headers, 'mail', 3, 'additional_headers');
+            $headers = rtrim($headers);
+
+            return '' === $headers ? null : $headers;
+        }
+        if (Variable::TYPE_ARRAY !== $arg->type) {
+            throw new \TypeError(\sprintf(
+                'mail(): Argument #4 ($additional_headers) must be of type array|string, %s given',
+                self::valueTypeName($arg)
+            ));
+        }
+
+        return self::buildHeadersFromArray($arg->toArray());
+    }
+
+    /**
+     * php_mail_build_headers() — php-src ext/standard/mail.c (#21432).
+     *
+     * @throws \TypeError
+     * @throws \ValueError
+     */
+    public static function buildHeadersFromArray(HashTable $headers): ?string
+    {
+        $lines = [];
+        foreach ($headers->iterateKeyed(true) as [$keyVar, $val]) {
+            $keyVar = $keyVar->resolveIndirect();
+            if (Variable::TYPE_INTEGER === $keyVar->type
+                || (Variable::TYPE_STRING === $keyVar->type && ctype_digit($keyVar->toString()))) {
+                $numeric = Variable::TYPE_INTEGER === $keyVar->type
+                    ? (string) $keyVar->toInt()
+                    : $keyVar->toString();
+                throw new \TypeError(\sprintf(
+                    'Header name cannot be numeric, %s given',
+                    $numeric
+                ));
+            }
+            if (Variable::TYPE_STRING !== $keyVar->type) {
+                throw new \TypeError(\sprintf(
+                    'Header name cannot be numeric, %s given',
+                    self::valueTypeName($keyVar)
+                ));
+            }
+            $name = $keyVar->toString();
+            $lower = strtolower($name);
+            if ('to' === $lower) {
+                throw new \ValueError('The additional headers cannot contain the "To" header');
+            }
+            if ('subject' === $lower) {
+                throw new \ValueError('The additional headers cannot contain the "Subject" header');
+            }
+            $val = $val->resolveIndirect();
+            if (Variable::TYPE_STRING === $val->type) {
+                $lines[] = self::formatHeaderLine($name, $val->toString());
+            } elseif (Variable::TYPE_ARRAY === $val->type) {
+                if (isset(self::STRING_ONLY_HEADERS[$lower])) {
+                    throw new \TypeError(\sprintf(
+                        'Header "%s" must be of type string, array given',
+                        $lower
+                    ));
+                }
+                foreach ($val->toArray()->iterateKeyed(true) as [$subKeyVar, $subVal]) {
+                    $subKeyVar = $subKeyVar->resolveIndirect();
+                    if (Variable::TYPE_STRING === $subKeyVar->type && !ctype_digit($subKeyVar->toString())) {
+                        throw new \TypeError(\sprintf(
+                            'Header "%s" must only contain numeric keys, "%s" found',
+                            $name,
+                            $subKeyVar->toString()
+                        ));
+                    }
+                    if (Variable::TYPE_INTEGER !== $subKeyVar->type
+                        && Variable::TYPE_STRING !== $subKeyVar->type) {
+                        throw new \TypeError(\sprintf(
+                            'Header "%s" must only contain numeric keys, "%s" found',
+                            $name,
+                            self::valueTypeName($subKeyVar)
+                        ));
+                    }
+                    $subVal = $subVal->resolveIndirect();
+                    if (Variable::TYPE_STRING !== $subVal->type) {
+                        throw new \TypeError(\sprintf(
+                            'Header "%s" must only contain values of type string, %s found',
+                            $name,
+                            self::valueTypeName($subVal)
+                        ));
+                    }
+                    $lines[] = self::formatHeaderLine($name, $subVal->toString());
+                }
+            } else {
+                throw new \TypeError(\sprintf(
+                    'Header "%s" must be of type array|string, %s given',
+                    $name,
+                    self::valueTypeName($val)
+                ));
+            }
+        }
+        if ([] === $lines) {
+            return null;
+        }
+
+        return implode("\r\n", $lines);
+    }
+
+    /**
+     * php_mail_build_headers_elem() field name/value checks.
+     *
+     * @throws \ValueError
+     */
+    private static function formatHeaderLine(string $name, string $value): string
+    {
+        if (!self::isValidHeaderName($name)) {
+            throw new \ValueError(\sprintf(
+                'Header name "%s" contains invalid characters',
+                $name
+            ));
+        }
+        self::assertValidHeaderValue($name, $value);
+
+        return $name.': '.$value;
+    }
+
+    private static function isValidHeaderName(string $name): bool
+    {
+        $n = \strlen($name);
+        for ($i = 0; $i < $n; ++$i) {
+            $ord = \ord($name[$i]);
+            if ($ord < 33 || $ord > 126 || ':' === $name[$i]) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    /**
+     * @throws \ValueError
+     */
+    private static function assertValidHeaderValue(string $name, string $value): void
+    {
+        $n = \strlen($value);
+        $i = 0;
+        while ($i < $n) {
+            $ch = $value[$i];
+            if ("\0" === $ch) {
+                throw new \ValueError(\sprintf(
+                    'Header "%s" contains NULL character that is not allowed in the header',
+                    $name
+                ));
+            }
+            if ("\r" === $ch) {
+                if (($i + 1) >= $n || "\n" !== $value[$i + 1]) {
+                    throw new \ValueError(\sprintf(
+                        'Header "%s" contains CR character that is not allowed in the header',
+                        $name
+                    ));
+                }
+                if (($i + 2) < $n && (' ' === $value[$i + 2] || "\t" === $value[$i + 2])) {
+                    $i += 3;
+                    continue;
+                }
+                throw new \ValueError(\sprintf(
+                    'Header "%s" contains CRLF characters that are used as a line separator and are not allowed in the header',
+                    $name
+                ));
+            }
+            if ("\n" === $ch) {
+                if (($i + 1) < $n && (' ' === $value[$i + 1] || "\t" === $value[$i + 1])) {
+                    $i += 2;
+                    continue;
+                }
+                throw new \ValueError(\sprintf(
+                    'Header "%s" contains LF character that is not allowed in the header',
+                    $name
+                ));
+            }
+            ++$i;
+        }
+    }
+
+    private static function valueTypeName(Variable $var): string
+    {
+        if (EnumCaseSupport::isEnumCaseVariable($var)) {
+            return EnumCaseSupport::typeNameForVariable($var);
+        }
+
+        return match ($var->type) {
+            Variable::TYPE_NULL => 'null',
+            Variable::TYPE_BOOLEAN => 'bool',
+            Variable::TYPE_INTEGER => 'int',
+            Variable::TYPE_FLOAT => 'float',
+            Variable::TYPE_STRING => 'string',
+            Variable::TYPE_ARRAY => 'array',
+            Variable::TYPE_OBJECT => 'object',
+            default => 'unknown type',
+        };
+    }
+
     /**
      * Deliver via INI `sendmail_path` (mirrored host / `-d` / PHP_COMPILER_INI_SENDMAIL_PATH).
      */
diff --git a/ext/standard/mail.php b/ext/standard/mail.php
index 392ec3ba66a..c53891cc4e5 100644
--- a/ext/standard/mail.php
+++ b/ext/standard/mail.php
@@ -40,8 +40,7 @@ public function execute(Frame $frame): void
         $message = VmString::coercePathBuiltinArg($frame->calledArgs[2], 'mail', 2, 'message');
         $headers = null;
         if ($argc >= 4) {
-            $headers = VmString::coerceStringBuiltinArg($frame->calledArgs[3], 'mail', 3, 'additional_headers');
-            VmString::rejectNullByteBuiltinStringArg($headers, 'mail', 3, 'additional_headers');
+            $headers = VmMail::coerceAdditionalHeaders($frame->calledArgs[3]);
         }
         $extraParams = null;
         if (5 === $argc) {
diff --git a/test/compliance/cases/stdlib/mail_headers_array.phpt b/test/compliance/cases/stdlib/mail_headers_array.phpt
new file mode 100644
index 00000000000..a1ba096af28
--- /dev/null
+++ b/test/compliance/cases/stdlib/mail_headers_array.phpt
@@ -0,0 +1,44 @@
+--TEST--
+stdlib mail() additional_headers array|string + ValueError (#21432, ext/standard/mail.c)
+--INI--
+sendmail_path={PWD}/mail_fixtures/mock_sendmail.sh
+--FILE--
+<?php
+$mock = ini_get('sendmail_path');
+$out = dirname($mock) . '/mock_sendmail.last';
+@unlink($out);
+
+try {
+    mail('a@b.c', 'subj', 'body', ['To' => 'evil@x']);
+    echo "TO_NO_THROW\n";
+} catch (ValueError $e) {
+    echo (str_contains($e->getMessage(), '"To"') ? 'to_value_error' : 'to_other'), "\n";
+}
+
+try {
+    mail('a@b.c', 'subj', 'body', ['Subject' => 'x']);
+    echo "SUBJ_NO_THROW\n";
+} catch (ValueError $e) {
+    echo (str_contains($e->getMessage(), '"Subject"') ? 'subj_value_error' : 'subj_other'), "\n";
+}
+
+try {
+    mail('a@b.c', 'subj', 'body', ['From' => "a\nb"]);
+    echo "LF_NO_THROW\n";
+} catch (ValueError $e) {
+    echo (str_contains($e->getMessage(), 'LF') ? 'lf_value_error' : 'lf_other'), "\n";
+}
+
+@unlink($out);
+$ok = mail('user@example.com', 'Hello', "Body\n", ['From' => 'noreply@example.com']);
+var_export($ok);
+echo "\n";
+$raw = is_file($out) ? file_get_contents($out) : '';
+echo (str_contains($raw, 'From: noreply@example.com') ? 'has_from' : 'no_from'), "\n";
+@unlink($out);
+--EXPECT--
+to_value_error
+subj_value_error
+lf_value_error
+true
+has_from
diff --git a/test/repro/issue_21432_mail_headers_array.php b/test/repro/issue_21432_mail_headers_array.php
new file mode 100644
index 00000000000..3c07f5944b9
--- /dev/null
+++ b/test/repro/issue_21432_mail_headers_array.php
@@ -0,0 +1,18 @@
+<?php
+
+/**
+ * Repro #21432 — mail() array additional_headers ValueError parity.
+ *
+ *   php bin/vm.php -d sendmail_path=…/mock_sendmail.sh test/repro/issue_21432_mail_headers_array.php
+ */
+try {
+    mail('a@b.c', 'subj', 'body', ['To' => 'evil@x']);
+    fwrite(STDERR, "NO_THROW\n");
+    exit(1);
+} catch (ValueError $e) {
+    if (!str_contains($e->getMessage(), '"To"')) {
+        fwrite(STDERR, "BAD_MSG: ".$e->getMessage()."\n");
+        exit(1);
+    }
+}
+echo "OK\n";