That will depend on a couple of things. Mail send will normally be gone from the server as soon as it is delivered, unless it is caught by the daily backup before delivering, then it sits in the backup tape. ISP's can access any mails on the server, but will only do it if there is a court order to do so, or if ordered by the owner of that mailbox.

Webmail, like hotmail works a bit different. It wil sit on the server untill deleted. Same goes with your send messages, it wil stay on the server until you delete it from your send box.

Rest assure, that no one, including ISP's may access your mail without a court order, or without your permission, unless there is a legitimate abuse report filed against you, that needs investigation.