Accessed from Configure 
Preferences Spelling
Currently there is no facility to do a spell check on a completed message: spellchecking is done automatically as you type.
When enabled, 
words will be automatically underlined in red.
Right-clicking on a word produces a context menu with suggestions of the correct spelling of what might be the intended word.
Click a Hot Spot on the image below for further information.
If autocorrection is enabled, misspellings contained in the built-in list will be automatically corrected, without user intervention - it will also be impossible to enter such misspellings without their being auto-corrected !
The list of commonly mis-typed words and their correct versions is held in a file called CORRECT.TLX, held (typically) in C:\PROGRAM FILES\MAILCOPA\SPELL. This may be edited by hand (at your own risk!) to include your own entries.
They are of the form:
Misspelling <TAB> Acorrect version
eg:
teh Athe
Warning: If you frequently mistype (for example) the word from as form, do not be tempted to enter this combination, or you will not be able to enter the word form without its being corrected to from !