For those of you who are using Gmail, you should do back ups just in case something goes wrong, such as your account is hacked, or something wreaks havoc with Googles storage, or if you just feel the need to have your stuff safe at home on your computer.
There are two ways of backing up your emails. The first method is to use Google to create an archive and store it somewhere out there in the cloud. When you are finished, you get an email with a link so you can download your back up to your computer as well. The instructions can be found at https://www.wikihow.com/Back-Up-Your-Gmail-Account.
The second is more complex and results in a backup just on your computer. Follow the instructions at https://github.com/jay0lee/got-your-back/wiki. It involves downloading some software written specifically for Gmail. Therefore it is written to handle all the tags etc used in Gmail. I believe I have read somewhere it is recommended by Google Gmail as the preferred back up software.
You don't only have to do one. You can use both.