ISPs regularly tweak their spam filters to try to protect their customers from excessive spam, and often trap wanted email in the process. While Bigpond's tech support team says your provider, whether it be ONTRAPORT or any other service, "sends a lot of spam out and has a bad reputation" the real cause is that they need to tweak their filters a bit more.
We checked, and in September, 2018, ONTRAPORT email to Bigpond servers is being delivered at 97.28% with only about 2.72% rejected back to us with a spam header. And our seed testing that is done daily still shows 100% delivery to the inbox on Bigpond servers for our "warmed" addresses. If we were really "sending out a lot of spam" and had a "bad reputation" their filters would be bouncing back our emails like we were a Nigerian prince.
How to Restore Delivery to the Inbox:
1. Start by contacting ONTRAPORT support - you can chat them by using the question mark "help" icon at the lower right of your account - and let them know you want us to check delivery to bigpond addresses.
2. Set up SPF and DKIM for your domain that you use as the "send from" address. It has to be a "regular" domain and not a gmail, yahoo or hotmail email address. See this article for details on doing this: https://support.ontraport.com/hc/en-us/articles/217880638-Remove-via-ontramail-com-From-Your-Email-FROM-Address
3. After you have SPF and DKIM set up, contact bigpond and ask them to help. One of our clients had success when they emailed their postmaster this note:
"We are experiencing issues in delivering emails to our customers who are using @bigpond email addresses. Customers are reporting that emails sent from our domain (@yourdomain.com) go directly to their spam folder and are only accessible logging in via the online login.
This is having a significant business impact as these customers cannot access information that they paid for. We seem to be able to email directly from our inbox, but not when sending emails via a bulk mail out.
Can you advise steps we can take to rectify this issue?"
In this case, Bigpond responded that they added a "bypass for your address into the antispam engine so this should not happen again."
Here's the postmaster's address (this forum won't allow it put in as a mailto link):