diff --git a/test/Fixtures/FileIsAccessibleTest/accessible.txt b/test/Fixtures/FileIsAccessibleTest/accessible.txt
new file mode 100644
index 00000000..6227a9dc
--- /dev/null
+++ b/test/Fixtures/FileIsAccessibleTest/accessible.txt
@@ -0,0 +1 @@
+Testing 1-2-3
\ No newline at end of file
diff --git a/test/Fixtures/FileIsAccessibleTest/inaccessible.txt b/test/Fixtures/FileIsAccessibleTest/inaccessible.txt
new file mode 100644
index 00000000..6227a9dc
--- /dev/null
+++ b/test/Fixtures/FileIsAccessibleTest/inaccessible.txt
@@ -0,0 +1 @@
+Testing 1-2-3
\ No newline at end of file
diff --git a/test/PHPMailer/FileIsAccessibleTest.php b/test/PHPMailer/FileIsAccessibleTest.php
new file mode 100644
index 00000000..1a1b0929
--- /dev/null
+++ b/test/PHPMailer/FileIsAccessibleTest.php
@@ -0,0 +1,104 @@
+<?php
+
+/**
+ * PHPMailer - PHP email transport unit tests.
+ * PHP version 5.5.
+ *
+ * @author    Marcus Bointon <phpmailer@synchromedia.co.uk>
+ * @author    Andy Prevost
+ * @copyright 2012 - 2020 Marcus Bointon
+ * @copyright 2004 - 2009 Andy Prevost
+ * @license   http://www.gnu.org/copyleft/lesser.html GNU Lesser General Public License
+ */
+
+namespace PHPMailer\Test\PHPMailer;
+
+use PHPMailer\PHPMailer\PHPMailer;
+use ReflectionMethod;
+use Yoast\PHPUnitPolyfills\TestCases\TestCase;
+
+/**
+ * Test file accessibility verification functionality.
+ *
+ * {@internal There is one test case known to be missing, which is a test
+ * with a valid UNC path. If someone can figure out a way to add a test for
+ * that case, that would be awesome!}
+ *
+ * @covers \PHPMailer\PHPMailer\PHPMailer::fileIsAccessible
+ */
+final class FileIsAccessibleTest extends TestCase
+{
+
+    /**
+     * Verify whether the "is a file accessible" check works correctly.
+     *
+     * @dataProvider dataFileIsAccessible
+     *
+     * @param string $input    A relative or absolute path to a file.
+     * @param bool   $expected The expected function return value.
+     */
+    public function testFileIsAccessible($input, $expected)
+    {
+        $reflMethod = new ReflectionMethod(PHPMailer::class, 'fileIsAccessible');
+        $reflMethod->setAccessible(true);
+        $result = $reflMethod->invoke(null, $input);
+        $reflMethod->setAccessible(false);
+
+        self::assertSame($expected, $result);
+    }
+
+    /**
+     * Data provider.
+     *
+     * @return array
+     */
+    public function dataFileIsAccessible()
+    {
+        $fixturesPath = dirname(__DIR__) . '/Fixtures/FileIsAccessibleTest/';
+
+        return [
+            'Valid: accessible file' => [
+                'input'    => $fixturesPath . 'accessible.txt',
+                'expected' => true,
+            ],
+            'Invalid: path not permitted' => [
+                'input'    => 'https://github.com/PHPMailer/PHPMailer/',
+                'expected' => false,
+            ],
+            'Invalid: file does not exist' => [
+                'input'    => $fixturesPath . 'thisfiledoesnotexist.txt',
+                'expected' => false,
+            ],
+            'Invalid: file in UNC path does not exist' => [
+                'input'    => '\\\\nowhere\nothing',
+                'expected' => false,
+            ],
+        ];
+    }
+
+    /**
+     * Test that the "is a file accessible" check correctly fails when the file permissions make
+     * the file unreadable.
+     */
+    public function testFileIsAccessibleFailsOnUnreadableFile()
+    {
+        if (\DIRECTORY_SEPARATOR === '\\') {
+            // Windows does not respect chmod permissions.
+            $this->markTestSkipped('This test requires a non-Windows OS.');
+        }
+
+        $path = dirname(__DIR__) . '/Fixtures/FileIsAccessibleTest/';
+        $file = $path . 'inaccessible.txt';
+        chmod($file, octdec('0'));
+
+        $reflMethod = new ReflectionMethod(PHPMailer::class, 'fileIsAccessible');
+        $reflMethod->setAccessible(true);
+        $result = $reflMethod->invoke(null, $file);
+        $reflMethod->setAccessible(false);
+
+        // Reset to the default for git files before running assertions.
+        chmod($file, octdec('644'));
+
+        self::assertFalse($result);
+    }
+}