Dear John,
According to http://www.directadmin.com/features.php?id=982
it says the 'allow leading plus in email names' feature has been added to the 1.337 version. But it doesn't work.
I still get
'String contains an invalid email address: +316229*****@sms.gin.nl'
Are you sure you got version 1.337?I did create that as Forwarder.