How to back up email
If you are managing email using POP3 Protocol then you are most likely storing all your mail on one device - your desktop computer.
You run the risk of losing all your stored email if your computer is damaged or stolen.
In this instance, regularly backing up of your email to an external drive will reduce this risk.
The method used will depend upon which email client you are using.
You should refer to the user guide for your email client.
When you use IMAP Protocol, your email is stored on the Mail Server.
Whilst hosting companies perform regular backups, these backups will not be available to you if and when you close your hosting account with them.
It is therefore good practice to periodically back up your email to your computer's Email Client.
And from there to an external drive (see POP above).
How to back up mail when using IMAP Protocol
It is very easy to back up mail form the server to your computer.
But be aware that when you do back up email you are physically moving it from the Mail Server to your computer.