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

In 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.

White envelope for Incoming Email

Blue envelope for Outgoing Email

Where an email is part of a thread, the icon indicates the type of the most recent email.

Refer also to Document Shortcuts and Managing Email Threads.