I receive emails at a .co.za address - so I don't think it's your .nz address that's the issue. I've had problems with authorisation emails not coming through from a variety of places but it's because the spam filters in hosted emails are removing those emails as spam, not because the sites aren't sending them.
You can try whitelisting *@itch.io on your server (anything from the address @itch.io)
or
You create a free email from yahoo or gmail and use that to create your account and just set those emails to forward to your normal address and then ask itch.io to change your email address afterwards.