Backing up your mail for safety If you wish to have a copy of all your mail on your local computer in case you ever lose access to your online gmail account, you should use a mail client to download the mail over POP or IMAP. You need to use a mail client so that, should you lose access to gmail, you’ll have a client that is capable of reading your stored mail.
The simplest technique is to download all your mail using a POP client as described in the Gmail help page. You can use pretty-much any mail client, but Mozilla Thunderbird or MS Outlook/Windows mail are good choices.
Using a POP client will grab all the mail into one big folder. If you want to preserve the folder/label structure you use in gmail, or you want to select only certain mail to download offline, you’ll need to use an IMAP client instead. If you use this technique, make sure the IMAP client is configured to give access to all the content offline. By default, many clients download only headers of the mails and not their full content.