Comment 10 for bug 893992

If you find that the envelope does not turn blue for all messages but only occasionally, this is probably not a bug but a feature, see Bug #957922 and #841116. Thunderbird now uses some algorithm to decide which (supposedly important) emails should turn the icon blue and which not, Since the user is not aware of this, the behavior may appear as random. This is possibly a duplicate of either #957922 or #841116.