When calling imap_close($mbox), notices will be generated for each error that has occurred within the imap functions. To suppress these error messages (including Mailbox is empty, which is not really an error) simply call imap_errors() and then imap_close($mbox).
(PHP 4, PHP 5)
imap_errors — Returns all of the IMAP errors that have occured
Gets all of the IMAP errors (if any) that have occurred during this page request or since the error stack was reset.
When imap_errors() is called, the error stack is subsequently cleared.
This function returns an array of all of the IMAP error messages generated since the last imap_errors() call, or the beginning of the page. Returns FALSE if no error messages are available.