Unable to connect to Exchange server connection to server failed . iPhone

An iPhone has previously been connecting to Exchange email without any problems. Two days ago it stopped, generating an error:
Cannot get mail. The connection to the server failed.
- I confirmed the password was correct.
- I confirmed that webmail was working.
- I confirmed the phone was connected to the carrier, Telstra.
- I deleted and recreated the account, but this issue remained.
- I created my own account on the iphone, and received the same error.
Settings > General > About says:
- Version: 4.2.1 (8C148)
- Model: MB489X
- Firmware: 05.15.04
Any ideas how to resolve this?
MB489X iOS 4