You can use an external spelling checker command to verify the spelling of you message. The default command used is ispell. You can change the default spelling checker command in the Composition Window Preferences dialog.
To use the spelling checker on the current message in the Composition window, click the Check spelling ... option in the Message pull down menu. The spelling checker checks the spelling of the current message.
ispell is quite easy to use. When ispell has checked the message for spelling errors, it displays a separate window in which misspelled words are highlighted in context. In that window you can instruct ispell's menu interface to accept a current spelling, replace a word with the correctly spelled word, insert a word, look up the spelling of another word, or capitalize/uncapitalize a word.
For detailed information about ispell, refer to the appropriate UNIX manual reference page.