bonkas: I beleive when using send.xtra.co.nz each account sending through it need to be authorised. You used to do this by logging into the main xtra account (one used for authentication) and allowing another users email address.

The process is outlined here: http://help.spark.co.nz/app/answers/detail/a_id/1547/~/verifying-third-party-email-addresses-in-yahoo-mail

I install stunnel to allow the SmartHost to connect the send.xtra.co.nz (SSL). Then used the updated process here to successfully add another allowed send address. Note: you have to switch to the "basic" view to allow the addition of addresses.