I have tested with GMail, (pop.gmail.com) if i get all message from pop server, for a certain time interval or till i receive new mail, i can't get all mails again (i mean form the same client, if i try to get mails more than once i get this issue).

I have tried to configure my GMail box but there are no change, is that behaviour an aspect of POP3's protocol or is a GMail's policy? 
I have the same issue to my customer's mailbox and would like to solve it, is there a way to configure his mail box so that i can get all mails every time i looking for them? 

If no is the answer, i haven't choice and i need to change my businsess logic, but if possibile i would like to avoid it.

PS : the command i use to check if there are mail to download (this command didn't work the second time i invoke it) is the command STAT