I am also having this problem and I am trying to send a private message to you with the details requested in the solution at the top of this thread
However I have just joined this forum and don't have sufficient privileges to send a pm. What can I do?
Thank you Telstra support. Problem with rejected emails solved for our websites bulk emails (myopengarden.com.au & igarden.com.au).
It seems that Telstra's anti-spam vendor had blacklisted our emails and they had to get our websites off their list. (It's a bit puzzling that the error says suspect content but heh ..)
In fairness to Telstra once I had sent the details via PM to them they did resolve the issues albeit over a couple of weeks. (I waited a couple of days since the "fix" to make sure our regular email dispatch went out early Monday morning - all good)