The CheckMailbox method requests that the server create a
checkpoint of the currently selected mailbox, and updates the
current number of new, unread messages available to the client.
When the client requests a checkpoint, the server may perform
implementation-dependent housekeeping for that mailbox, such
updating the mailbox on disk with the current state of the mailbox
in memory. On some systems this command has no effect other than to
update the client with the current number of messages in the
mailbox.
This function actually sends two IMAP commands. The first is the
CHECK command, followed by the NOOP command to poll for any new
messages that have arrived. In addition to polling the server for
new messages, this command can also be used to ensure the idle
timer on the server does not expire and force a disconnect from the
client.