What is the difference between the blue and white envelope icons that display in lists for emails?

In Document Lists, an icon displays in the left-hand column to show the type of document. For emails, this icon is an envelope. The colour of the icon indicates if the email is incoming or outgoing.

Where an email is part of a thread, the icon displayed will depend on the most recent email in the thread.

Refer also to Document Types Overview and Managing Email Threads.