Hi Bargains,
You can try our troubleshooting tool here https://www.telstra.com.au/webforms/cares/
This has a Email support guide to try and assist in diagnosing and repairing common email account issues.
You can also try removing your email profile from outlook then re-adding it, you can find help on how to do this here alnog with other support tools https://www.telstra.com.au/support/category/email
If you still are having difficulties after trying the above, we would be happy to assist you with further troubleshooting this for you to see what is causing this issue. Please speak to our Technical support team. You can contact them on 133933 or you can also contact them via our Live chat service here: http://tel.st/49kl
Regards -