Expected: An email arrives, and Thunderbird updates its Icon marker (in Windows) to have a "1" on it (or increases it by 1). When that email is marked as read, Thunderbird then either reduces the count by 1, or removes it if it's no longer needed. What happens: If the email is deleted (or archived on GMail) without the unread marker on it being set to "read" then the email vanished from my inbox, but the counter is not changed. What should happen: If the email is deleted/ moved out of a folder that Thunderbird watches, into one it doesn't, then the unread count should be updated.