If you use Outlook 2007 and plain text format, be aware there is a bug in "Always check spelling before sending". The spell check on send appears to work but in reviewing the Sent item and the copy the recipient receives, the misspelled words remain misspelled.

This only happens on plain text messages, not HTML or RTF. Correcting the errors as you type or by doing a manual spelling check (F7) works as expected; only the automatic check initiated when you press Send doesn't work.