Open Windows Mail.

On the Main toolbar, Click the Tools option.

From the drop down menu that appears, Select Options.

The Option window will now appear.

In the Options Window, Click the Spelling tab.

Under the 'Settings' section at the top of the window, Place a Check Mark in the 'always check spelling before sending' option.

Make any alterations you need to make under the 'when checking spelling, always ignore' option. For instance you may not wish to have the original text in replies or forwards checked for spelling errors, in which case you should place a check mark in the 'original text in a reply or forward' option.

Next look near the bottom of the window under the Language section and make sure that the correct language is selected.

Finally Click OK to close the Options Window.

Now every time you send an email, Windows Mail will automatically check the contents for spelling errors.