diff --git a/CHANGELOG b/CHANGELOG
index a6db41741..077e27fcb 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -17,6 +17,7 @@ CHANGELOG Roundcube Webmail
 - Indicate that a collapsed thread has flagged children (#5013)
 - Implemented message/rfc822 attachment preview
 - Update to jsTimezoneDetect 1.0.6
+- Managesieve: Support 'string' test from variables extension [RFC 5229] (#5248)
 - Managesieve: Support 'duplicate' extension [RFC 7352]
 - Managesieve: Unhide advanced rule controls if there are inputs with errors
 - Managesieve: Display warning message when filter form contains errors
diff --git a/plugins/managesieve/Changelog b/plugins/managesieve/Changelog
index 8fa7ac88f..5e1042307 100644
--- a/plugins/managesieve/Changelog
+++ b/plugins/managesieve/Changelog
@@ -1,5 +1,6 @@
-* version 8.7 [2016-]
+* version 8.7 [2016-07-19]
 -----------------------------------------------------------
+- Support 'string' test from variables extension [RFC 5229] (#5248)
 - Support 'duplicate' extension [RFC 7352]
 - Unhide advanced rule controls if there are inputs with errors
 - Display warning message when filter form contains errors
diff --git a/plugins/managesieve/lib/Roundcube/rcube_sieve_engine.php b/plugins/managesieve/lib/Roundcube/rcube_sieve_engine.php
index 5f5a6b072..6fc702729 100644
--- a/plugins/managesieve/lib/Roundcube/rcube_sieve_engine.php
+++ b/plugins/managesieve/lib/Roundcube/rcube_sieve_engine.php
@@ -1451,14 +1451,14 @@ class rcube_sieve_engine
         if (in_array('body', $this->exts)) {
             $select_header->add($this->plugin->gettext('body'), 'body');
         }
-        if (in_array('variables', $this->exts)) {
-            $select_header->add($this->plugin->gettext('string'), 'string');
-        }
         $select_header->add($this->plugin->gettext('size'), 'size');
         if (in_array('date', $this->exts)) {
             $select_header->add($this->plugin->gettext('datetest'), 'date');
             $select_header->add($this->plugin->gettext('currdate'), 'currentdate');
         }
+        if (in_array('variables', $this->exts)) {
+            $select_header->add($this->plugin->gettext('string'), 'string');
+        }
         if (in_array('duplicate', $this->exts)) {
             $select_header->add($this->plugin->gettext('message'), 'message');
         }
diff --git a/plugins/managesieve/managesieve.js b/plugins/managesieve/managesieve.js
index cfc60db7f..6cb5b3865 100644
--- a/plugins/managesieve/managesieve.js
+++ b/plugins/managesieve/managesieve.js
@@ -608,6 +608,7 @@ function rule_header_select(id)
     comp = document.getElementById('rule_comp' + id),
     datepart = document.getElementById('rule_date_part' + id),
     dateheader = document.getElementById('rule_date_header_div' + id),
+    rule = $('#rule_op' + id),
     h = obj.value;
 
   if (h == 'size') {
@@ -634,11 +635,14 @@ function rule_header_select(id)
   if (dateheader)
     dateheader.style.display = h == 'date' ? '' : 'none';
 
+  $('[value="exists"],[value="notexists"]', rule).prop('disabled', h == 'string');
+  if (!rule.val() || rule.val().match(/^(exists|notexists)$/))
+    rule.val('contains');
+
   rule_op_select(op, id, h);
   rule_mod_select(id, h);
-  
-  if (h == '...') obj.style.width = '40px';
-  else if (h == 'string') obj.style.width = '90px';
+
+  obj.style.width = h == '...' ? '40px' : '';
 };
 
 function rule_op_select(obj, id, header)
@@ -904,24 +908,27 @@ function sieve_formattime(hour, minutes)
 
 function sieve_form_init()
 {
-  // small resize for header element
-  $('select[name="_header[]"]', rcmail.gui_objects.sieveform).each(function() {
-    if (this.value == '...') this.style.width = '40px';
-    if (this.value == 'string') this.style.width = '90px';
-  });
+  var form = rcmail.gui_objects.sieveform;
 
   // resize dialog window
   if (rcmail.env.action == 'plugin.managesieve' && rcmail.env.task == 'mail') {
-    parent.rcmail.managesieve_dialog_resize(rcmail.gui_objects.sieveform);
+    parent.rcmail.managesieve_dialog_resize(form);
   }
 
-  $('input[type="text"]:first', rcmail.gui_objects.sieveform).focus();
+  $('input[type="text"]:first', form).focus();
 
   // initialize smart list inputs
-  $('textarea[data-type="list"]', rcmail.gui_objects.sieveform).each(function() {
+  $('textarea[data-type="list"]', form).each(function() {
     smart_field_init(this);
   });
 
+  // initialize rules form(s)
+  $('[name="_header[]"]', form).each(function() {
+    if (/([0-9]+)$/.test(this.id)) {
+      rule_header_select(RegExp.$1);
+    }
+  });
+
   // enable date pickers on date fields
   if ($.datepicker && rcmail.env.date_format) {
     $.datepicker.setDefaults({
diff --git a/plugins/managesieve/tests/src/parser_variables b/plugins/managesieve/tests/src/parser_variables
index bd5941c02..c4ea2f72a 100644
--- a/plugins/managesieve/tests/src/parser_variables
+++ b/plugins/managesieve/tests/src/parser_variables
@@ -10,3 +10,15 @@ set :lower "b" "${a}";
 set :upperfirst "b" "${a}";
 set :upperfirst :lower "b" "${a}";
 set :quotewildcard "b" "Rock*";
+if string :matches " ${state} " "* pending *"
+{
+	set "test1" "*";
+}
+if string ["aaa","bbb"] ["aaa","bbb"]
+{
+	set "test2" "*";
+}
+if string :is :comparator "i;octet" "bbb" "bbb"
+{
+	set "test3" "*";
+}