Add a "Personal Preferences Profile" in the e-mail template with a field to enable/disable automatic spell checking. Then, modify the Send & Send & File actions in the Memo; Reply; & Reply with History forms with the following code (add it to the beginning of the other code in the actions):
speller:=@GetProfileField("PersonalProfile"; "AutoSpellCheck"); @If(speller="Yes";@Command([EditGotoField];"Body") & @Command([EditSelectAll]) & @Command([ToolsSpellCheck]);"");