diff --git a/test/Fixtures/HasLineLongerThanMaxTest/cert.pem b/test/Fixtures/HasLineLongerThanMaxTest/cert.pem
new file mode 100644
index 00000000..e7c90b30
--- /dev/null
+++ b/test/Fixtures/HasLineLongerThanMaxTest/cert.pem
@@ -0,0 +1,31 @@
+-----BEGIN CERTIFICATE-----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+W9UGX8HtU1LB94dWoVWNvISFvjicCWhVsuNw1Z0tIko499iEQG+zezbmh++n9a2G
+bkCaI6dFZL5pHakmKOTYKyZ1sprE4799KDSTd8hlPfHboC4ClWqIiI6ou3kEpJln
+sdsNZP5vPHrDgjuW/oE+zsQjmkaJiWaZphpthyYkR32Xu7HPvtQT4MHfkrs/SFE0
+43ml8CqrGSa+IjSjI+HXMwsf0mRmEtK7PcqVLhdSWAGjMNPjJ+er/O+PX3ZAWrbl
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+0ZksoB/XpdHRaqTRF0h6G2EUXznOG7x04uG4tiHkim1W4IkBBVTLxp6iul9n8GAe
+QoZadHGaPIeytwl7A986Qo78WIltxZC+SBjJeQJG7/qHt/MvB8dBXZ49zg1SmHeV
+ZtBWdtC0LBGcLoImm9m7DCyA9xMqSKSoOqmzXTlWcKQnPi3MeI5dqfWzuvk9LyLg
+71hXXF4EnTgZpHw1ZWJBI47jEfsH2G0c7X46HPYjD4XcDCChNG81d0xQpZMbq5J7
+jy6PSbE/iGghEOuiF1NpQsrAnlf0UAzA27bUPyX0NFOmQmAejc8b6NqIljSQE/Xm
+MOZlE4RpIa63EzzDo1fas8hhUhtz3loYysHN+nmw4N0BRjVenxatJXiunIbsDkzJ
+VeBTrddQlqszd0qOlGCpJM0uhHuVDPntHKmFLo7O32aH2TgvYakpp54Xd+4xIqM=
+-----END CERTIFICATE-----
diff --git a/test/Fixtures/HasLineLongerThanMaxTest/key.pem b/test/Fixtures/HasLineLongerThanMaxTest/key.pem
new file mode 100644
index 00000000..6d7ec066
--- /dev/null
+++ b/test/Fixtures/HasLineLongerThanMaxTest/key.pem
@@ -0,0 +1,52 @@
+-----BEGIN PRIVATE KEY-----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+DATtfJg5aUXxy/MmlEebHrG0onidu3YZPel8Yj6JY0P7j1lSUcGiXvm6zgpNWk3p
+wpWD1KFIc8lJdeSsWtGs1f1SEUkzbjZu/TwHMJfVxY2GWqVsHtiTiDitsbgWhxVX
+Th8Vd12yq7DfjxhHO0ZkobUDaOPem32FrnjVyWf/ZEDAOLTZpeycXnCQWEqU2R7T
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++3OgeIntFG1RMspltJTMtJgPhExTkB2yf78jUFrtV7wGCZDDkKDkB+aLkGH0eygT
+M+doZSJgFy2fvdYxmzMCggEBAPuQzgO6ab7zfGcaHhlbhXpHl9twMQefYGdH/DVx
+yS91Ef1ygUsEgAICNIYAX6bvaBBL6akTf9kMbixo8j47KM6o6uR2ogze9z4nnKUk
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+axwlVHoCG+XhiwlUZGD4tD8L0qjzWIn+T/0T4kKAEAvTleG/KZNkCRbOS3mycwrM
+ouJGGkdapt5Fh/cmnv+lO5wh5QbkYaJS/kpUkYGljy1/s3HTlvDsw955dADeH1+0
++/2nwfK54dgoB8m037AyIqrwKY6dhGDES7cTUZ5AqiAtJ7MCggEBAMqRFV9YbyBg
+Ktgni4Q/vXLCqsaZN7JVnKxjiCqbpJ8UZuGv4Ifalq8npwo0634Hfi10p/54TKxy
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+lAktAzB4KuvIrCRMedSO6m+bYpq5P89Vbq2Ovl7fwJWvYDAEUUe7hhkrUDDOxHu/
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+csaWPGYkEa8CggEBAN3zjrBfYjklXlchBflddFDEpcjoHXoaNdYp/u2wbM7APZUd
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+Uo8/9qN9gE4sCYTvbnZ4MTTZIGygkD+mYVYcN6nol0ZQQqDNwckLV+OiGnCExxm2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+-----END PRIVATE KEY-----
diff --git a/test/PHPMailer/HasLineLongerThanMaxTest.php b/test/PHPMailer/HasLineLongerThanMaxTest.php
index af6118d6..f9618df9 100644
--- a/test/PHPMailer/HasLineLongerThanMaxTest.php
+++ b/test/PHPMailer/HasLineLongerThanMaxTest.php
@@ -56,6 +56,46 @@ final class HasLineLongerThanMaxTest extends PreSendTestCase
         );
     }
 
+    /**
+     * Test constructing a SMIME signed message that contains lines that are too long for RFC compliance.
+     *
+     * @covers \PHPMailer\PHPMailer\PHPMailer::hasLineLongerThanMax
+     */
+    public function testLongBodySmime()
+    {
+        $oklen = str_repeat(str_repeat('0', PHPMailer::MAX_LINE_LENGTH) . PHPMailer::getLE(), 2);
+        // Use +2 to ensure line length is over limit - LE may only be 1 char.
+        $badlen = str_repeat(str_repeat('1', PHPMailer::MAX_LINE_LENGTH + 2) . PHPMailer::getLE(), 2);
+
+        $this->Mail->Body = 'This message contains lines that are too long.' .
+            PHPMailer::getLE() . $oklen . $badlen . $oklen;
+        self::assertTrue(
+            PHPMailer::hasLineLongerThanMax($this->Mail->Body),
+            'Test content does not contain long lines!'
+        );
+
+        $this->Mail->isHTML();
+        $this->buildBody();
+        #$this->Mail->AltBody = $this->Mail->Body;
+        $this->Mail->Encoding = '8bit';
+        $this->Mail->sign(
+            __DIR__.'/../Fixtures/HasLineLongerThanMaxTest/cert.pem',
+            __DIR__.'/../Fixtures/HasLineLongerThanMaxTest/key.pem',
+            null,
+        );
+        $this->Mail->preSend();
+        $message = $this->Mail->getSentMIMEMessage();
+        self::assertFalse(
+            PHPMailer::hasLineLongerThanMax($message),
+            'Long line not corrected (Max: ' . (PHPMailer::MAX_LINE_LENGTH + strlen(PHPMailer::getLE())) . ' chars)'
+        );
+        self::assertStringContainsString(
+            'Content-Transfer-Encoding: quoted-printable',
+            $message,
+            'Long line did not cause transfer encoding switch.'
+        );
+    }
+
     /**
      * Test constructing a message that does NOT contain lines that are too long for RFC compliance.
      *