The POP3 protocol (used to retrieve email) can either be configured to leave mail on the server or delete mail once downloaded.
Most people set their mail software to delete mail once downloaded in order not to have to download the same mail items every time. Once the mail server receives an instruction to delete mail, that item is permanently deleted - it does not go into a deleted items folder or recycle bin because the POP3 protocol has no concept of that.
Our (most IPSs) backup procedures perform continues backups, but, since we process several gigabytes and tens of thousands of email per day, we cannot keep the backups indefinitely - so we have to implement for some kind of limit - after all, no computing device has infinite capacity.
There is also a privacy issue: It can be argued that an ISP should not be storing items a user explicitly deleted - other than guarding against accidental deletion and hardware failure on our servers of course.
The second problem is that the POP3 (and all mail servers using this standard protocol) does not use "transactions" - this means that we can only restore to a certain point of time...like a snapshot. A frozen image.
This means that if we restore your mailbox to the state it was 14 days ago, you'll lose all email and changes made since - exactly the same way a backup on your own PC works. If you restore to a point, everything done after that is lost...which is the point of a backup - to restore a previous state.
Having explained the problems in backing up email, here are the solutions/recommendations:
1. If you use Outlook on your computer, you can set the software to automatically archive your email for as long as you like and at the intervals you need. This is the most efficient way of backing up your email. Most email software has this capability. Best practice would be to dump all your backups onto a separate (removable) disk drive.
2. If you use the IMAP protocol (instead of POP3) your email software will sync with the server. If your computer's disk crashes, you will be able to re-sync to the server. However, you still will not be able to selectively restore email for specific days. The only reliable way to do that is to use an archive file on your own computer.
So - the important thing to remember is that your should NOT rely on your ISP to backup your mail as they can do only so much for you. Make sure you protect your own information. It is irreplaceble.