Evolution Ctrl-] doesn't jump across mailboxes.  This behavior is a positive
feature when compared to Ctrl-n in Thunderbird which can easily jump to another
mailbox and confuse the user.  However, adding a seperate keybing, Shft-Ctrl-]
(Ctrl-}) for example, to explicitly jump to the next unread in the next mailbox
would similarly prevent user confusion while speeding up reading mail in
multiple mailboxes.