When setting up an email account on your computer you have 2 choices: POP or IMAP.
POP is the original mail protocol. Use this if you want to store the emails on your computer instead of the server. This is handy if you have limited server space.
You are responsible for backing up your inbox on your mail program (e.g. outlook) regularly. If you delete your email account on your computer, you may lose your inbox and sent messages.
POP doesn’t sync with the server and your other devices. If you reply to an email on your computer, you will not see this on your mobile device. This protocol simply downloads the emails from the server to your computer.
IMAP is a modern protocol. This keeps all sent and received emails on the server and syncs them with your computer and devices.
If you delete your mailbox, or setup the account on a different device, everything can be restored. If you reply to an email on your computer, you will see that reply on your devices.
IMAP uses a lot more storage space on the server, so can be more costly. The more emails you use the more space you will require, which will means upgrading your hosting plan or manually deleting emails.