He is using Vista and Windows Mail but I don't think they are the issue.
It seems whenever he sends out mail occasionally the mail send fails with some sort of authorisation error. He spent hours on the phone with Xtra support who had him remove the account, add it back and all kinds of other changes to no avail.
Finally he called me and I setup a teamviewer session to take a look.
From the Windows Mail client, mail to certain addresses (one of them in the xtra domain no less) was getting blocked by the Yahoo SMTP server with some sort of security error. It looked like it was a e-mail blacklist but it seems hardly likely that Yahoo would block mail coming from an xtra domain (he is using send.xtra.co.nz as the SMTP server).
We then tried using the web client. The same failure occurred.
And then this is the strange one, when we tried to send to another account (not Xtra) up popped a Captura type prompt asking for characters to be entered before the mail would go out. Once entered the mail went out fine.
I don't use Xtra myself (nor Yahoo) so didn't have any personal experience to draw on but I have never seen this sort of behaviour before. I see that Xtra e-mails have been hacked again so perhaps his account was identified by the Yahoo servers as being the sender of SPAM or something so his e-mail address is being blocked, not the domain itself? Of course I have no way to check that but perhaps Telecom staff might?
Thanks. He is finding it very annoying.
I did think about having him create a Gmail account (or he might have one) and using the Gmail SMTP server. That should work but the mail comes out with the header sent by email@example.com on behalf of firstname.lastname@example.org which can be confusing for some.