This is normally a configuration issue with your MS Outlook e-mail program.
To rectify this:
While Outlook is open, go to the Tools menu and choose E-mail Accounts (or Account Settings if using Outlook 2007 or newer). If you are prompted on what you want to do, choose to edit an e-mail account (not applicable to Outlook 2007 or newer).
Choose your e-mail address and click "Change". From there, you will see the settings for your e-mail account.
First, make sure both the Incoming (POP3) and Outgoing (SMTP) servers are set to mail.your-domain-name.com
Secondly, click the "More settings" button. On the screen that pops up, click on the "Advanced" tab. Uncheck "This server requires an encrypted connection (SSL)" and make sure the Outgoing (SMTP) port number is either 25 or 26 (26 is most likely to work for you, as 25 is often blocked by internet service providers).
Finally, after doing that, click back to the "Outgoing Server" tab and make sure "My Outgoing Server (SMTP) requires authentication" is checked, and "use same settings as my incoming mail server" is selected. If present, also make sure to select "None" from the pull down menu for "Use the following type of encryption connection".
With the above changes, you will have no problems sending e-mails from Outlook.