diff --git a/CHANGELOG b/CHANGELOG
index 81558ad4c46..9ba086ebe13 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -4,6 +4,7 @@ CHANGELOG Roundcube Webmail
 - Password: Fix compatibility with PHP 7+ in cpanel_webmail driver (#5820)
 - Fix decoding non-ascii attachment names from TNEF attachments (#5646, #5799)
 - Fix uninitialized string offset in rcube_utils::bin2ascii() and make sure rcube_utils::random_bytes() result has always requested length (#5788)
+- Fix bug where HTML messages with @media styles could moddify style of page body (#5811)
 
 RELEASE 1.3.0
 -------------
diff --git a/program/lib/Roundcube/rcube_utils.php b/program/lib/Roundcube/rcube_utils.php
index 3c0fc1eb224..cc524470274 100644
--- a/program/lib/Roundcube/rcube_utils.php
+++ b/program/lib/Roundcube/rcube_utils.php
@@ -430,7 +430,10 @@ public static function mod_css_styles($source, $container_id, $allow_remote = fa
         $source = preg_replace(
             array(
                 '/(^\s*<\!--)|(-->\s*$)/m',
-                '/(^\s*|,\s*|\}\s*)([a-z0-9\._#\*][a-z0-9\.\-_]*)/im',
+                // (?!##str) below is to not match with ##str_replacement_0##
+                // from rcube_string_replacer used above, this is needed for
+                // cases like @media { body { position: fixed; } } (#5811)
+                '/(^\s*|,\s*|\}\s*|\{\s*)((?!##str)[a-z0-9\._#\*][a-z0-9\.\-_]*)/im',
                 '/'.preg_quote($container_id, '/').'\s+body/i',
             ),
             array(
diff --git a/tests/Framework/Utils.php b/tests/Framework/Utils.php
index 71e9f3e30fb..9b71acef5d6 100644
--- a/tests/Framework/Utils.php
+++ b/tests/Framework/Utils.php
@@ -183,6 +183,7 @@ function test_mod_css_styles()
 
         $this->assertContains('#rcmbody table[class=w600]', $mod, 'Replace styles nested in @media block');
         $this->assertContains('#rcmbody {width:600px', $mod, 'Replace body selector nested in @media block');
+        $this->assertContains('#rcmbody {min-width:474px', $mod, 'Replace body selector nested in @media block (#5811)');
     }
 
     /**
diff --git a/tests/src/media.css b/tests/src/media.css
index 24eacc8a186..34e5b434251 100644
--- a/tests/src/media.css
+++ b/tests/src/media.css
@@ -15,6 +15,7 @@ div, p, a, li, td { -webkit-text-size-adjust:none; }
 	body {width:600px !important; margin:auto !important;}
 	.pict img {max-width:540px !important;  height:auto !important;}
 }
+@media screen and (max-width:600px) { body {min-width:474px !important;} }
 h1{ font-weight:bold; font-size:14px;color:#3c3c3c ;margin:0px; }
 h2{ color:#8DB048 ; font-size:14px; font-weight:bold; margin-top:20px; border-bottom:1px solid #d6d6d6; padding-bottom:4px; }
 h3{ color:#7e7e7e ; font-size:14px; font-weight:bold; margin:20px 0px 0px 0px; border-bottom:1px solid #d6d6d6; padding-bottom:0px 0px 4px 0px; }