However, for backing up emails,one would like to keep the email even if it is deleted from the server. How to achieve this?
You may use `getmail` which can retrieve emails through pop3 and imap4 and save the emails to maildirs and mboxrd files.
getmail is a mail retriever designed to allow you to get your mail from one or more mail accounts on various mail servers to your local machine for reading with a minimum of fuss. getmail is designed to be secure, flexible, reliable, and easy-to-use. getmail is designed to replace other mail retrievers such as fetchmail.
You can use
procmail to fetch email from pop3 which is usually supported by email servers and store the email in Maildir.
There are lots good tutorials on the Web which you can refer to:
Backup your Gmail account in maildir format using fetchmail: http://blog.wains.be/2008/03/19/backup-your-gmail-account-in-maildir-format-using-fetchmail/
Managing your email with fetchmail, procmail and mutt: http://easierbuntu.blogspot.com/2011/09/managing-your-email-with-fetchmail.html
Using fetchmail and procmail for maildir style storage from a pop3 account: http://www.ioncannon.net/system-administration/97/using-fetchmail-and-procmail-for-maildir-style-storage-from-a-pop3-account/
Geek to Live: Back up Gmail with fetchmail: http://lifehacker.com/235207/geek-to-live–back-up-gmail-with-fetchmail