I could guess this way. If the letter sent by post is undelivered and returned to CHC, they will try to contact you in the alternate address, if any, like e mail, fax or phone. If still no response from you, then the Immigration Section will make a note on the file that "the The letter sent to PA is undelivered and cannot be contacted by e mail, phone or fax." As a result your file will be on hold for certain period or till they get your proper contact details.