I see that this issue doesn't appear to have been discussed here. However if Xnet customers weren't already aware, if you send email via smtp.xnet.co.nz to an Xtra/Yahoo/ Telecom email address, they more than likely will not receive the email, as Yahoos spam filtering appear to have blacklisted Xnets IP numbers.
This is a major problem as so many NZers use Xtra/Yahoo/Telecom email addesses. The emails first get delayed by many hours, and then they go into yahoos bulk mail filter. If the xtra customer doesn't use Yahoo webmail, and only downloads their email using POP, then they will never receive the email that is in the bulk mail bin.
Xnet are fully aware of the problem and apparently have been in contact with Yahoo to fix the problem. However this problem isn't posted on their network status page, which would be handy if they could, then we can know when the problem has been fixed. Apparently the problem is also affecting other ISPs, not just email that is sent from Xnet. This problem appears to have gone on for at least a week, and Xnet don't appear to have yet been able to get their IPs off Yahoos blacklist.
I have however independently contacted Yahoo via their Bulk Email Abuse form, and managed to get the IP address I have been temporarily assigned by Xnet off the blacklist, but Yahoo refuse to take the other IPs off the black list. If I reset my modem however, my IP address will be reset, and I will again be blacklisted.
This issue is also being discussed here http://pressf1.co.nz/showthread.php?t=84221
And there are also quite a few articles on the internet about this problem.