Some ISPs in Australia require their customers use their outbound mail servers and block access to any DreamHost SMTP servers. Telstra Bigpond is a popular example.
Telstra Bigpond blocks the use of any outbound mail servers other than their own. Many hotels and motels in Australia use Telstra Bigpond for their in-house WiFi.
View the following link for details on POP/IMAP/SMTP settings: