Exim error: R=dkim_lookuphost T=dkim_remote_smtp defer (-46) H=hostname [xx.xx.xx.xx]: SMTP error from remote mail server after end of data: 451 Local Error or something like this: dkim_lookuphost T=dkim_remote_smtp defer (111): Connection refused

If you are getting this error on cPanel server, you should try disable dkim on your existing users accounts. To do this you can in your WHM panel or using API.