EssentialPIM provides a built-in spell checker. You can spell check by pressing F7 or under Tools -> Spelling check menu.
This will call a dialog which proposes general spelling check features:
Ignore - ignores current occurrence of the word
Ignore All - ignores all occurrences of the word
Change - changes the word to a selected variant
Change All - changes all occurrences of the word to a selected variant
Add - adds word to a dictionary
Dictionaries - allows to change dictionaries
Spell check options are available under Tools -> Options -> Spelling
English (US) dictionary is installed by default but you can add any additional dictionaries you need. To do so, click on the checkbox besides the dictionary's name in the Available dictionaries section and click on the Proceed button. EPIM might ask you for administrator rights during the process, please confirm. Once installation is complete, new dictionaries will become instantly available.
To uninstall a dictionary, uncheck the checkbox near its name in the Available dictionaries section and click on the Proceed button.
To simply disable a dictionary, uncheck the checkbox near its name in the Installed dictionaries section.