Majority of email users use web-based email services such as Gmail, Outlook, or Yahoo. This simply means that your email messages are stored on their servers and are hardly ever downloaded to your PC, except of course, you have previously done so.
The implication is that if anything happens to your email account or email service provider, you might lose all your email data. It might, therefore, be a smarter move to also keep a copy of your valuable email messages as back up.
If you will, therefore, like to back up your web-based email, then, the first step would be for you to check if your service provider has enabled that option at the back end.
If such option is available, then, you can use POP (Post Office Protocol) Mail with a third party email client. To do this, you will have to switch to POP3 in your webmail’s settings. You can, then, setup your email account with a program like Mozilla Thunderbird on the desktop. This will download not only your emails, but also the folders and categories too. Thunderbird is open source and 100% free to use.
There is need for you to make provisions for this kind of back up to avoid total loss of your data in case anything happens to the servers of your web-based service provider.