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.
Messageware Plus Pack makes OWA resemble Outlook by seamlessly integrating essential features most requested by users familiar with Outlook, thereby enhancing usability and user productivity. Some key features include intuitive addressing from public folders and SharePoint lists, rolodex-style address book, roaming personal dictionaries, advanced searching and calendar views/reminders, and an interactive new mail notification that allows the user to see/read/reply/forward/delete new messages.
Speckie for Outlook is an add-in for Microsoft Office Outlook that integrates Speckie in to any window that displays webpages throughout Outlook. This results in the only real-time, dedicated spell checking solution available for products such as Microsoft Dynamics CRM Outlook Client. Speckie for Outlook works with Microsoft Office Outlook 2003, 2007 and 2010 on Windows XP, Vista and 7. Available for both the 32 and 64 bit versions of Outlook.
Most download sites can point you toward additional spelling dictionaries for Office and generic Windows spelling tools.