Microsoft Outlook and the full Exchange Server client have spell checking built in. Microsoft Exchange Server 2003, Exchange 2007, and Exchange 2010 include spell checking for Outlook Web Access (OWA).
Microsoft Office 2007 users are missing spell check dictionaries in Outlook Express. If you use OE and your only option for spelling is French, see Office 2007 and OE Spell Check for more information and a work-around.
More Information
Most download sites can point you toward additional spelling dictionaries for Office and generic Windows spelling tools.
disconnect the spell check feature in mail forwarding
Outlook has an option to ignore the original on replies and forwards - if you add a note, that will be spellchecked.