The Apple Mail.app in Mavericks is a terrible step backwards.
Smart mailboxes behave flakily – I’ve got one that’s called “All unread” (it’s rule is simply “message is unread”) and it said that there were 5 messages unread, until I click on it and suddenly there’s 21 unread messages. This is true of any Smart Mailbox, as best as I can tell – it won’t report the new number of unread mail that match the Smart Mailbox rule until you select it. This defeats the point! The Mail.App in 10.8 used to report this number accurately, long before I selected the mail box.
Also if I login to Gmail’s web interface, I can see that I have three unread messages sitting there. None of these messages have turned up in my Gmail inbox in Mail.app. I tried synchronising mailboxes, Get All New Mail, nothing seems to make it turn up.
The old Apple.mail I had in 10.8 was reliable. This one IS NOT. It’s damaged, and awful.
See my prior struggles — previously all overcome — with Mail.App here.
Update: just before I hit save on this blog post the three unread messages (plus one that I read earlier in the Gmail web app) turned up in my Mail.app inbox. The earliest received was more than three hours ago, and i’ve been sitting here with the Mail.app open for an hour and a half. And now I’ve written this paragraph the computer finally made the ping alert for new mail. Seriously, screw that s–t.