To use spell checking for the emails that you write you first need to make sure that the hunspell package for your specific language and the enchant package are installed via the software management tool of your distribution.

Install hunspell now.

Install enchant now.

Under EditPreferencesComposer PreferencesSpell CheckingOptions you can define whether your spelling is checked while you type and which color is used for underlining words that are misspelled.

You can also define which installed languages are used for spell checking in the list available under EditPreferencesComposer PreferencesSpell CheckingLanguages.

If you do not have Checking spelling while I type enabled in the Composer Preferences you can run a spell check in the email composer by clicking EditSpell Checking or by pressing F7.