I am using Thunderbird as an email client for a POP3 server. Recently, I formatted my hard disk drive to change the OS from Windows XP to Windows 7. Consequently, I lost all my local emails. I also don't remember whether or not I had the server settings "Leave messages on server Until I delete them". Is it possible to retrieve back those old messages ?
Manoj,
You need to contact your email hosting provider to see what's possible on their end. Many don't keep permanent copies of emails. Once a customer downloads the email and deletes it, it is gone. Even if they make daily backups, it's possible that than an email arrived during the day and you downloaded it and deleted it before a backup ever happened.
If these messages are very important to you, you may have some hope of extracting them from your hard drive, depending on how the formatting worked. You can study options for forensics tools or contact a professional data recovery service.