Check Disabled Items List
If Outlook has a problem loading, it will automatically disable its add in programs, including LetterMark. To fix this, locate the disabled items list, re-enable LetterMark and close and reopen Outlook.
The disabled items list in Outlook 2003 is located in the Help menu, About Microsoft Office Outlook, Disabled Items.
The disabled items list in Outlook 2007 is located in the Help menu.