How can mailbox quota limit messages impact server performance?

just a comment, if i remember well then in the past Outlook cashed Exchange, (outlook) forms. There was even a method to retrieve a lost form on exchange back trough a client. So stopped outlook doing this? or is something else happening because the article mentions that outlook is asking Exchange for this form (why is that needed ??? as outlook would have a cashed version of it, after it had opened once such form)