messenger plus has nothing to do with emails. messenger plus is an addon for msn messenger.
if you want to spell check your hotmail emails before sending them, then click tools > spell check (or just spell check, if using windows live hotmail) at the top of the page, near the send button
edit:
if you want a spell checker for plus! then he's an add on that includes a spell checker
http://www.msgpluslive.net/scripts/view/132-Etcetera/
haven't used it myself tho, so can't tell you how good it is
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