offlineimap actually does “synchronization”—if an email is deleted from the server, the email will be deleted from the local Maildir (http://offlineimap.org/howitworks.html). This is good for mail sync.
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 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