diff --git a/api/js/etemplate/et2_widget_url.js b/api/js/etemplate/et2_widget_url.js
index 65c8becb6f..ffa5d6dd51 100644
--- a/api/js/etemplate/et2_widget_url.js
+++ b/api/js/etemplate/et2_widget_url.js
@@ -41,12 +41,14 @@ var et2_url = (function(){ "use strict"; return et2_textbox.extend(
 	 * - "Becker < Ralf <rb@stylite.de>" (contains <    ----------- " ---------------)
 	 *
 	 * About umlaut or IDN domains: we currently only allow German umlauts in domain part!
+	 * We forbid all non-ascii chars in local part, as Horde does not yet support SMTPUTF8 extension (rfc6531)
+	 * and we get a "SMTP server does not support internationalized header data" error otherwise.
 	 *
 	 * Using \042 instead of " to NOT stall minifyer!
 	 *
 	 * Same preg is in Etemplate\Widget\Url PHP class!
 	 */
-	EMAIL_PREG: new RegExp(/^(([^\042',<][^,<]+|\042[^\042]+\042|\'[^\']+\'|)\s?<)?[^\x00-\x20()<>@,;:\042\[\]]+@([a-z0-9ÄÖÜäöüß](|[a-z0-9ÄÖÜäöüß_-]*[a-z0-9ÄÖÜäöüß])\.)+[a-z]{2,}>?$/i),
+	EMAIL_PREG: new RegExp(/^(([^\042',<][^,<]+|\042[^\042]+\042|\'[^\']+\'|)\s?<)?[^\x00-\x20()<>@,;:\042\[\]\x80-\xff]+@([a-z0-9ÄÖÜäöüß](|[a-z0-9ÄÖÜäöüß_-]*[a-z0-9ÄÖÜäöüß])\.)+[a-z]{2,}>?$/i),
 	/**
 	 * @memberOf et2_url
 	 */
diff --git a/api/src/Etemplate/Widget/Url.php b/api/src/Etemplate/Widget/Url.php
index 7b532efb5b..25dd76a1f7 100644
--- a/api/src/Etemplate/Widget/Url.php
+++ b/api/src/Etemplate/Widget/Url.php
@@ -35,10 +35,12 @@ class Url extends Etemplate\Widget
 	 * - ToDo: "\u200Bfrancesca.klein@ikem.de" (starts with a "zero width space")
 	 *
 	 * About umlaut or IDN domains: we currently only allow German umlauts in domain part!
+	 * We forbid all non-ascii chars in local part, as Horde does not yet support SMTPUTF8 extension (rfc6531)
+	 * and we get a "SMTP server does not support internationalized header data" error otherwise.
 	 *
 	 * Same preg is in et2_widget_url Javascript class, but no \x00 allowed and /u modifier for utf8!
 	 */
-	const EMAIL_PREG = "/^(([^\042',<][^,<]+|\042[^\042]+\042|\'[^\']+\'|)\s?<)?[^\x01-\x20()<>@,;:\042\[\]]+@([a-z0-9ÄÖÜäöüß](|[a-z0-9ÄÖÜäöüß_-]*[a-z0-9ÄÖÜäöüß])\.)+[a-z]{2,}>?$/iu";
+	const EMAIL_PREG = "/^(([^\042',<][^,<]+|\042[^\042]+\042|\'[^\']+\'|)\s?<)?[^\x01-\x20()<>@,;:\042\[\]\x80-\xff]+@([a-z0-9ÄÖÜäöüß](|[a-z0-9ÄÖÜäöüß_-]*[a-z0-9ÄÖÜäöüß])\.)+[a-z]{2,}>?$/iu";
 
 	/**
 	 * Validate input