Hi
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.
Actually if the letter is returned or no response to emails, then they close the file.