POP3 and IMAP are protocols used to received emails from a mail server in a very different way.
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POP3 is intended to work like this. Your mail client connects to a mail server and download the email to local computer/device and then delete the emails from the mail server. This protocol is older and was used back in days when a user did not have a permanent connection to the internet.
There are additional settings to save the emails on the server after they are downloaded to the local computer. This protocol is not used in the situation when a user wants to check the emails of the same mailbox using few different devices, because it just does not suit this purpose. IMAP will not show sent emails from one device when you check the same mailbox on the other device. Also, if you delete the email from a local computer, the email will not be deleted from the server (if the saving option is enabled)
IMAP is an alternative protocol, which suits modern user needs. Assuming that user has the internet connection almost anytime now, the mail client on the local computer is constantly synchronizing information between the mail client and the mail server. When using IMAP your emails are not being downloaded to the local computer and rather stay on the mail server, your mail client only displays the emails.
This protocol is more suited for nowadays user because the same mailbox is now being checked on multiple devices. It allows the user to always check all emails, including sent emails. Whatever you do on your local mail client, it actually is done on the server, so the user can check, send or delete emails and the same result will be seen on all the devices using the same – IMAP protocol.
SMTP is more common nowadays due to the ability to check the same mailbox on multiple devices. However, keep in mind that all the work is done on servers, so in some cases, you may have an issue with this protocol if your mailbox has a limited disk quota.
POP3 is still used by some people, who only check mailbox from one device and has a limited mail server disk space.
Note. If you want to check the same mailbox on few different devices, do not ever use different protocols for each of your devices.