Message content rejected due to suspected spam. OB703'

I am having problems with sending emails with an error ’ Message content rejected due to suspected spam. OB703’  I have windows 10 pro, and email account is a bigpond.com account. The account has been set up with the IMAP and SMTP settings provided by Telstra. I have also uninstalled and re-installed the latest version of eM Client, and disabled my firewall/antivirus. I have also tried deleting my signature to test as well.

So this is not something new with Telstra. Probably better you contact them.