Copy and paste your password to ensure it is correct.
Most problems are due to incorrect settings.
Here are some online instructions for both Windows and Mac.
If you are using the secure settings, port 587 may work instead of port 465.
Mac Mail Tutorial
Apple Mac Mail issues
Note: Multiple login attempts with incorrect credentials will block your IP address. If your connection to the internet uses a dynamic IP address then you can resolve this issue by restarting (powercycling) your internet modem router.
IP blocking error by Telstra users
Try the following solutions.