diff --git a/htdocs/comm/action/index.php b/htdocs/comm/action/index.php
index e5ab1c02902..8327aedf9ce 100644
--- a/htdocs/comm/action/index.php
+++ b/htdocs/comm/action/index.php
@@ -1062,7 +1062,7 @@ if (count($listofextcals))
                     $event->ref = $event->id;
 
                     $userId = $userstatic->findUserIdByEmail($namecal);
-                    if(!empty($userId))
+                    if (!empty($userId) && $userId > 0)
                     {
                         $event->userassigned[$userId] = $userId;
                         $event->percentage = -1;
diff --git a/htdocs/user/class/user.class.php b/htdocs/user/class/user.class.php
index 6d278e635fa..42d2f8705fb 100644
--- a/htdocs/user/class/user.class.php
+++ b/htdocs/user/class/user.class.php
@@ -3240,42 +3240,62 @@ class User extends CommonObject
 
 	/**
 	 * Cache the SQL results of the function "findUserIdByEmail($email)"
-	 *
+	 * 
+	 * NOTE: findUserIdByEmailCache[...] === -1 means not found in database
+	 * 
 	 * @var array
 	 */
 	private $findUserIdByEmailCache;
 
 	/**
-	 * Find a user by the given e-mail or part of a e-mail and return its user id when found
+	 * Find a user by the given e-mail and return it's user id when found
 	 *
-	 * @param string	$email	The full e-mail or a part of a e-mail 
-	 * @return int|null			The id of the user when found, otherwise null
+	 * NOTE:
+	 * Use AGENDA_DISABLE_EXACT_USER_EMAIL_COMPARE_FOR_EXTERNAL_CALENDAR
+	 * to disable exact e-mail search
+	 *
+	 * @param string	$email	The full e-mail (or a part of a e-mail)
+	 * @return int				<0 = user was not found, >0 = The id of the user
 	 */
 	public function findUserIdByEmail($email)
 	{
-		if($this->findUserIdByEmailCache[$email])
+		if ($this->findUserIdByEmailCache[$email])
 		{
 			return $this->findUserIdByEmailCache[$email];
 		}
 
+		$this->findUserIdByEmailCache[$email] = -1;
+
+		global $conf;
+
 		$sql = 'SELECT rowid';
 		$sql .= ' FROM '.MAIN_DB_PREFIX.'user';
-		$sql .= ' WHERE email LIKE "%'.$email.'%"';
+		
+		if (!empty($conf->global->AGENDA_DISABLE_EXACT_USER_EMAIL_COMPARE_FOR_EXTERNAL_CALENDAR))
+		{
+			$sql .= ' WHERE email LIKE "%'.$email.'%"';
+		}
+		else
+		{
+			$sql .= ' WHERE email = "'.$email.'"';
+		}
+
+		$sql .= ' LIMIT 1';
 
 		$resql = $this->db->query($sql);
 		if (!$resql)
 		{
-			return 0;
+			return -1;
 		}
 
 		$obj = $this->db->fetch_object($resql);
 		if (!$obj)
 		{
-			return 0;
+			return -1;
 		}
 
-		$this->findUserIdByEmailCache[$email] = (int)$obj->rowid;
+		$this->findUserIdByEmailCache[$email] = (int) $obj->rowid;
 
-		return (int)$obj->rowid;
+		return $this->findUserIdByEmailCache[$email];
 	}
 }