What e-mail address is Manager specifying in the “from” field? Is it your ISP-issued e-mail address, or is it an address within your own domain? If it’s the latter, have you altered the SPF record for your domain to indicate that your ISP’s mail server is allowed to deliver mail on your behalf? Hint: add “
include:spf.westnet.com.au” to your existing SPF string.
Do you have a G-mail account to test with? Try sending an e-mail to your G-mail account then look at the headers (while viewing the message on Google’s webmail interface, click the three dots on the far right and choose “show original”). The headers will tell you how Google processed the message with regard to SPF and spam scoring, and the multiple “Received:” lines will show what path the message took.
If you can’t decypher what the headers say, send them to me in a private message through here and I’ll help you out.
I was originally going to say that you shouldn’t expect anyone to consistently receive messages sent through no-reply.io or MailGun as the former resides in Amazon netspace which is widely blocked throughout the world, and the latter is a recognised spam-for-hire service, suffering the same widespread blocking for similar reasons - but you’ve already eliminated that possible cause, and I highly doubt that iiNet’s outbound servers would be blocked by anyone.