Some ISP's block the SMTP Relay causing outgoing messages to bounce. If this happens to you, follow the instructions below by changing the outgoing SMTP port settings.

In the Menu bar click on "Tools" and then "Accounts".

Click the "Mail" tab.
Highlight the account name.
Click on the "Properties" button.

Click on the "Servers" tab.
Make sure "My Server Requires Authentication" is checked.
If you make a change click "Apply" at the bottom.
Click the "Settings" button at the bottom.

Check the box "Use same settings as my incoming mail server".
Click "OK".

Go to the"Advanced" tab.
Change the Outgoing Server Port setting from 25 to 26.
Click "OK".

Click "Close" to close the Internet Accounts box.

If you are still having problems, reboot your computer and try sending email again.

If you are still having problems click here for another alternative.