Comment 9 for bug 841116

Well, it's a bit more complicated than "paint blue only if the arrived message is flagged".

Why should the default be to turn it blue? Remember that we share the menu with real-time messaging clients, and having e-mails always turn the icon blue reduces the usefulness of the messaging menu to pretty much zero (as it masks real-time messages that you need to deal with now). The feedback I've had back has mostly suggested that we actually turn the icon blue far too much, so we want to make it less noisy in Precise.

In fact, the new version that I'll be uploading to precise soon is significantly more selective about what messages will turn the indicator blue, and basically boils down to:

if (priority >= high || ((flagged || (addressed_to_me && sent_from_human && !from_mailing_list)) && priority > low))) then turn_icon_blue()

Remember that even if we don't turn the icon blue, we still add the message to the "new message" count on the launcher icon.