We recently had an issue where one of our inboxes was nearly full. So we reached out and did some research to find the best method of reliably backing up a Gmail inbox and clearing it. We found that the best method is with getmail, a Linux utility that fetches mail through IMAP. It is very powerful and flexible, providing a lot of options. There are several different methods as well (outlined below).

Overall, we recommend using getmail. By trying several different approaches, getmail is by far the best. It is a little confusing at first, but not a hard pill to swallow:

How do you install getmail?

Ubuntu:

sudo apt-get install dovecot-imapd getmail4

Arch Linux:

pacman -S getmail

Centos:

yum install getmail*

Here are some resources for backing up gmail with Linux:
getmail:

• Matt Cutts explains how to backup gmail with getmail 
• Linode documentation goes over getmail setup
• Getmail documentation
• Configuring getmail

If you don’t like getmail there are several different options out there:
BagoMa:

• Ebixio blog explains how to back up gmail with bagoma
• Bagoma documentation

Fetchmail:

• Life hacker explains how to backup gmail with fetchmail
• Periodically backing up your email with fetchmail

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