One of our clients has their ADSL through Xtra but all their other services through us. Starting on Monday, they haven't been able to send any email, via smtp.xtra.co.nz. We are presuming, that their outbound addresses have contained their domain for which Xtra is neither the registrar nor the MX of.
You may find that the SMTP servers want authentication now - as per http://www.xtra.co.nz/help/0,,4127-3548773,00.html#4 You should just need to enable SMTP Authentication using their Xtra username/password to get things rolling again. Just remember Xtra DSL uses joe.bloggs.xadsl(a)xtra.co.nz for the DSL authentication, however the SMTP Auth is done on joe.bloggs(a)xtra.co.nz (it's the same username/password they'll use to manage their account online)