It's a View with Automatic Formatting - simple formatting rules can be
created using Tools, Organize or use the Define view dialog on the View menu
to create complex rules.
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Diane Poremsky [MVP - Outlook]
Author, Teach Yourself Outlook 2003 in 24 Hours
Coauthor, OneNote 2003 for Windows (Visual QuickStart Guide)
Author, Google and Other Search Engines (Visual QuickStart Guide)
Is this random or alway for the same sender(s) or
subject?, Check your rules you may have set for
organizing your mail. On Mac and PC you can have mail
color coded as you choose.