This problem is apparently causing emails sent via Spark / Xtra /Yahoos SMTP server, to bounce back with the following error
550 5.7.1 You are not authorized to send as email@example.com
Apparently this is due to Spark currently being in the process of moving to a new email provider.
A temporary fix for this is to send out your email directly via Yahoos SMTP server setting. So this means changing your outgoing SMTP server setting from send.xtra.co.nz, to smtp.mail.yahoo.com. In most cases this should work, at least temporarily, until Yahoo deactivate this, and until Spark come up with a local solution. We do also suggest contacting Spark and raising a ticket with them, as the more people that report this problem to them, the more likely they will fix it properly.
Apparently people are having other problems to with the Spark email service during the migration, so this fix above may not help with those problems.
If you use the Lounge Network for your email, you don’t need to worry
about any of this, as we do provide SMTP access on our premium hosting
plans. If you are reading this and want to host your domain based email
with a NZ based email provider, please contact us, and we can provide you with details on our NZ Domain based Email Hosting Services. We do also do Business grade NZ Web Hosting, and are one of New Zealands longest established NZ Web Hosting providers.