"Leave a copy of messages on server"
This is fine, but you must remember to clear the inbox down regularly - your ISP provides a finite limit on mailbox size - maybe only a few dozen MB, which is soon filled up. When your mailbox gets full you probably won't be able to receive and / or send messages until you empty it.
Your ISP might even have further penalties - e.g. account suspension or charges for excess storage - but that's unlikely.
SD