Is it correct for client to send "SUBSCRIBE mailbox/" command?
According to the IMAP protocol, it is not incorrect. Yes, that's a double negative, but in this case that has a significance.
The server has its own discretion to decide whether it is correct, and if so how it should be treated compared to "SUBSCRIBE mailbox".
This is not something that I would depend on if I were a client author.
-- Mark --
http://staff.washington.edu/mrc Science does not emerge from voting, party politics, or public debate. Si vis pacem, para bellum.