diff --git a/snappymail/v/0.0.0/app/libraries/MailSo/Mime/HeaderCollection.php b/snappymail/v/0.0.0/app/libraries/MailSo/Mime/HeaderCollection.php
index 5b3674dbe..2704909c9 100644
--- a/snappymail/v/0.0.0/app/libraries/MailSo/Mime/HeaderCollection.php
+++ b/snappymail/v/0.0.0/app/libraries/MailSo/Mime/HeaderCollection.php
@@ -136,14 +136,12 @@ class HeaderCollection extends \MailSo\Base\Collection
 					$sHeadersValue = \rtrim($sHeadersValue);
 				}
 
-				if ('=?' === \substr(\ltrim($sHeadersValue), 0, 2)) {
-					$sHeadersValue = \ltrim($sHeadersValue);
-				}
-
-				if ('=?' === \substr($sHeadersValue, 0, 2)) {
-					$sValue .= $sHeadersValue;
+				// RFC 5322 unfolding removes the line break and keeps the folding
+				// whitespace; RFC 2047 6.2 drops it only between two encoded words.
+				if ('=?' === \substr(\ltrim($sHeadersValue), 0, 2) && '?=' === \substr($sValue, -2)) {
+					$sValue .= \ltrim($sHeadersValue);
 				} else {
-					$sValue .= "\n".$sHeadersValue;
+					$sValue .= ' '.\ltrim($sHeadersValue);
 				}
 			} else {
 				if (null !== $sName) {
diff --git a/test/header-unfold.php b/test/header-unfold.php
new file mode 100644
index 000000000..4b33c8d2f
--- /dev/null
+++ b/test/header-unfold.php
@@ -0,0 +1,51 @@
+<?php
+
+// Run with: php test/header-unfold.php
+// RFC 5322 unfolding keeps the folding whitespace; RFC 2047 drops only the
+// whitespace BETWEEN two encoded words. A plain word followed by a folded
+// encoded word lost its space: "de vos\r\n =?utf-8?Q?=C3=A9quipements?="
+// was shown as "voséquipements" (Symfony Mailer folds subjects this way).
+define('APP_VERSION', 'test');
+spl_autoload_register(static function (string $class): void {
+	if (str_starts_with($class, 'SnappyMail\\')) {
+		$file = dirname(__DIR__).'/snappymail/v/0.0.0/app/libraries/snappymail/'.strtolower(str_replace('\\', '/', substr($class, 11))).'.php';
+	} else {
+		$file = dirname(__DIR__).'/snappymail/v/0.0.0/app/libraries/'.str_replace('\\', '/', $class).'.php';
+	}
+	if (is_file($file)) {
+		require_once $file;
+	}
+});
+
+function check(bool $condition, string $message): void
+{
+	if (!$condition) {
+		throw new RuntimeException($message);
+	}
+}
+
+function subject(string $raw): string
+{
+	$headers = new \MailSo\Mime\HeaderCollection("Subject: {$raw}\r\nX-Next: 1\r\n");
+	return (string) $headers->ValueByName('Subject');
+}
+
+$cases = [
+	// plain word, then a folded encoded word: the space stays
+	"Traiter les =?utf-8?Q?vuln=C3=A9rabilit=C3=A9s?= de vos\r\n =?utf-8?Q?=C3=A9quipements?= : plan de =?utf-8?Q?rem=C3=A9diation?= et\r\n checklist =?utf-8?Q?sign=C3=A9e?="
+		=> 'Traiter les vulnérabilités de vos équipements : plan de remédiation et checklist signée',
+	// two encoded words split across lines: no space between them (RFC 2047 §6.2)
+	"=?utf-8?Q?r=C3=A9pon?=\r\n =?utf-8?Q?se?=" => 'réponse',
+	"=?utf-8?B?w6l0w6k=?=\r\n\t=?utf-8?B?IGZpbg==?=" => 'été fin',
+	// encoded word, then a folded plain word: the space stays
+	"=?utf-8?Q?r=C3=A9union?=\r\n demain" => 'réunion demain',
+	// no encoding at all
+	"Hello\r\n world" => 'Hello world',
+];
+foreach ($cases as $raw => $want) {
+	// exact: a fold is one space, never two ("et  checklist" ended up in replies)
+	$got = subject($raw);
+	check($got === $want, "got [{$got}], want [{$want}]");
+}
+
+echo "header-unfold: ".count($cases)." checks OK\n";