1. Open Mail
  2. Go to the Mail menu and select Preferences...
  3. Select the Accounts tab
  4. Select the '+' button on the bottom left to add a new account
  5. Select Other Mail Account...
  6. Enter your Name, Email Address, Password and select Sign In
  7. Select POP or IMAP for 'Account Type' and enter 'mail.omni.net.au' for both Incoming Mail Server and Outgoing Mail Server and select Sign In
  8. Select Done

Note: Apple Mail should have configured the outgoing mail server correctly, but if you can't send mail follow these steps:
  1. Open Mail
  2. Go to the Mail menu and select Preferences
  3. Select the Accounts tab
  4. Select the account in the list on the left
  5. Under Outgoing Mail Server (SMTP), select Edit SMTP Server List... in the drop down menu
  6. Make sure the correct account is selected at the top part of the window and the Port is set to 587 and SSL is ticked and you have your username and password entered.