If you are getting an error similar to "550 not local host [domainname], not a gateway", or something similar, then it may be because your outgoing mail settings are not setup correctly.
You must enable the setting, Outgoing mail server requires authentication. Use the same login information as incoming mail.
Outlook 2000 / Outlook Express:
Open Outlook Express or Outlook 2000 (Depending on what version you may have on your computer).
Click on "Tools".
Select "Accounts" from the menu.
Select the account you would like to edit and click on "Properties".
Click on the "Servers" tab.
Click the check box labeled, "My Server Requires Authentication".
Click "OK".
Click "Close".
Outlook 2003 / Outlook 2007 / Outlook XP:
Open Outlook 2003, Outlook 2007 or Outlook XP (Depending on what version you may have on your computer)
Click on "Tools".
Click on "E-mail Accounts".
Select, "View or Change Existing E-Mail Accounts" and click "Next >"
Select the account you would like to make the change to and click, "Change..."
Click on "More Settings..." button.
Now select the "Outgoing Server" tab.
Check the box, "My outgoing mail server (SMTP) requires authentication".
Leave the option, "Use same settings as my incoming mail server" selected.
Click "OK".
Click "Next >".
Click "Finish".
Once you have made that change to ensure that you are being authenticated to send out mail, will will be able to send e-mail. Try these settings, if you continue to have problems, please see the related articles.