diff --git a/custom_from.php b/custom_from.php
index 28c6cb5..a2596c7 100644
--- a/custom_from.php
+++ b/custom_from.php
@@ -361,7 +361,7 @@ private static function get_configuration(rcmail $rcmail)
         $use_preference = !$rcmail->config->get('custom_from_preference_disable', false);
 
         // Read "contains" parameter from global configuration & preferences if allowed
-        $contains = $rcmail->config->get('custom_from_compose_contains', '');
+        $contains = $rcmail->config->get(self::PREFERENCE_COMPOSE_CONTAINS, '');
 
         if ($use_preference) {
             $contains = self::get_preference($rcmail, self::PREFERENCE_COMPOSE_CONTAINS, $contains);
diff --git a/tests/CustomFromTest.php b/tests/CustomFromTest.php
index 2571a2d..3c77852 100644
--- a/tests/CustomFromTest.php
+++ b/tests/CustomFromTest.php
@@ -12,48 +12,37 @@
 
 final class CustomFromTest extends TestCase
 {
+    const CONTAINS = 'custom_from_compose_contains';
+    const DISABLE = 'custom_from_preference_disable';
+    const RULES = 'custom_from_header_rules';
+    const SUBJECT = 'custom_from_compose_subject';
+
     public static function storage_init_should_fetch_headers_provider(): array
     {
         return array(
             array(
-                array(
-                    'custom_from_compose_auto' => false
-                ),
+                array('custom_from_compose_auto' => false),
                 array(),
                 ''
             ),
             array(
-                array(
-                    'custom_from_compose_auto' => true
-                ),
+                array('custom_from_compose_auto' => true),
                 array(),
                 'bcc cc from to x-original-to'
             ),
             array(
-                array(
-                    'custom_from_header_rules' => 'header1=a;header2=b'
-                ),
+                array(self::RULES => 'header1=a;header2=b'),
                 array(),
                 'header1 header2'
             ),
             array(
-                array(
-                    'custom_from_header_rules' => 'header=a',
-                    'custom_from_preference_disable' => true
-                ),
-                array(
-                    'custom_from_compose_subject' => 'always'
-                ),
+                array(self::RULES => 'header=a', self::DISABLE => true),
+                array(self::SUBJECT => 'always'),
                 'header'
             ),
             array(
-                array(
-                    'custom_from_header_rules' => 'header=a',
-                    'custom_from_preference_disable' => false
-                ),
-                array(
-                    'custom_from_compose_subject' => 'always'
-                ),
+                array(self::RULES => 'header=a', self::DISABLE => false),
+                array(self::SUBJECT => 'always'),
                 'header bcc cc from to x-original-to'
             )
         );
@@ -75,29 +64,106 @@ public function test_storage_init_should_fetch_headers($config_values, $user_pre
     public static function message_compose_should_set_state_provider(): array
     {
         return array(
+            // Subject rule "exact" should match address exactly
             array(
-                array('to' => 'primary@domain.ext'),
+                array('to' => 'alice@primary.ext'),
+                array(self::RULES => 'to=e'),
+                array(),
+                null, // TODO: assert alice@primary.ext was matched
+            ),
+            // Subject rule "exact" shouldn't match suffix
+            array(
+                array('to' => 'alice+suffix@primary.ext'),
+                array(self::RULES => 'to=e'),
+                array(),
                 null,
             ),
+            // Subject rule "prefix" should match address exactly
             array(
-                array('to' => 'primary+suffix@domain.ext'),
-                'primary+suffix@domain.ext',
-            )
+                array('to' => 'alice@primary.ext'),
+                array(),
+                array(),
+                null, // TODO: assert alice@primary.ext was matched
+            ),
+            // Subject rule "prefix" should match address by prefix
+            array(
+                array('to' => 'alice+suffix@primary.ext'),
+                array(),
+                array(),
+                'alice+suffix@primary.ext',
+            ),
+            // Subject rule "prefix" should not match different user
+            array(
+                array('to' => 'carl+suffix@primary.ext'),
+                array(),
+                array(),
+                null,
+            ),
+            // Subject rule "domain" on custom header should match address by domain
+            array(
+                array('to' => 'unknown@primary.ext', 'x-custom' => 'unknown@secondary.ext'),
+                array(self::RULES => 'x-custom=d'),
+                array(),
+                'unknown@secondary.ext' // TODO: assert bob@primary.ext was matched
+            ),
+            // Subject rule "domain" should not match different domain
+            array(
+                array('to' => 'unknown@unknown.ext'),
+                array(self::RULES => 'to=d'),
+                array(),
+                null,
+            ),
+            // Subject rule "other" should match anything
+            array(
+                array('to' => 'unknown@unknown.ext'),
+                array(self::RULES => 'to=o'),
+                array(),
+                'unknown@unknown.ext' // TODO: assert bob@primary.ext was matched
+            ),
+            // Subject rule is overridden by user prefrences
+            array(
+                array('to' => 'unknown@secondary.ext'),
+                array(self::RULES => 'to=e'),
+                array(self::SUBJECT => 'domain'),
+                'unknown@secondary.ext'
+            ),
+            // Contains constraint in configuration options matches address
+            array(
+                array('to' => 'alice+match@secondary.ext'),
+                array(self::CONTAINS => 'match'),
+                array(self::SUBJECT => 'domain'),
+                'alice+match@secondary.ext'
+            ),
+            // Contains constraint in configuration options rejects no match
+            array(
+                array('to' => 'alice+other@secondary.ext'),
+                array(self::CONTAINS => 'match'),
+                array(self::SUBJECT => 'domain'),
+                null
+            ),
+            // Contains constraint in user preferences rejects no match
+            array(
+                array('to' => 'alice+other@secondary.ext'),
+                array(self::CONTAINS => 'other'),
+                array(self::CONTAINS => 'match', self::SUBJECT => 'always'),
+                null
+            ),
         );
     }
 
     #[DataProvider('message_compose_should_set_state_provider')]
-    public function test_message_compose_should_set_state($message, $expected): void
+    public function test_message_compose_should_set_state($message, $config_values, $user_prefs, $expected): void
     {
-        $identity1 = array('email' => 'primary@domain.ext', 'name' => 'Primary', 'standard' => '1');
-        $identity2 = array('email' => 'secondary@domain.ext', 'name' => 'Secondary', 'standard' => '0');
+        $identity1 = array('email' => 'alice@primary.ext', 'name' => 'Alice', 'standard' => '0');
+        $identity2 = array('email' => 'bob@primary.ext', 'name' => 'Bob', 'standard' => '1');
+        $identity3 = array('email' => 'carl@secondary.ext', 'name' => 'Carl', 'standard' => '0');
 
         $id = 17;
         $uid = '42';
         $rcmail = rcmail::mock();
-        $rcmail->mock_config(array());
+        $rcmail->mock_config($config_values);
         $rcmail->mock_message($uid, $message);
-        $rcmail->mock_user(array($identity1, $identity2), array());
+        $rcmail->mock_user(array($identity1, $identity2, $identity3), $user_prefs);
 
         $plugin = self::create_plugin();
         $plugin->message_compose(array('id' => $id, 'param' => array('uid' => $uid)));