When marking an message as complete, mark it read and allow for it to clear categories (optional)
When marking an message as complete, mark the message as read and optionally allow for it to clear categories assigned to the message.
This is useful for those that use categories to as a more fine tune filtering option of the status of a project, task or idea.
Andrew Roley commented
Perhaps mage a quick step to mark complete, read, and Clear categories? Can't remember if the last one (categories) is possible...