Hm, some spam filters can outright reject "blatant spam" (in their evaluation) without even deposit it in the user's spam folder, so it could be that a common email provider in Australia has changed their spam filtering rules such that your thank-you emails are being mistakenly rejected as "blatant spam" right out of hand.
For this reason, we recommend keeping the thank-you email message as short, simple and to-the-point as possible, with as few URLs and promotional language as possible -- ideally, just using our default message and link for digital products (i.e., not using Templated Email) with little if any custom Email Message text added to that -- and save any promotional or instructional text for the thank-you/download page itself.
You may want to disable your item-specific thank-you emails entirely (Enable Templated Email but leave the Email Message field blank in the products' settings) and instead just send a single, Common Thank-you Email per order, which you can customize in Seller Admin > Account Preferences.
If you are also redirecting buyers to a Common Thank-you Page URL after checkout, so they must wait to receive an email to obtain their download, the [%thankyou_link%] included in the default Common Thank-you Email template we provide for you would insert a link that activates that same common URL redirection, so to prevent that, you would need to append &noredirect=true to the end of that code in your email template, like so:
[%thankyou_link%]&noredirect=true
