POP3 stands for Post Office Protocol 3, which is an older protocol designed to be used on only one computer. Unlike modern protocols that use two-way synchronization, POP3 only supports one-way email synchronization, only allowing users to download emails from a server to a client. This means that emails are tied to the specific device, and once the email is downloaded onto one device (and removed from the mail server), it cannot be accessed by another email client or device. POP3 accounts lack most of the basic functionality that can be found in more modern services, such as the ability to mark a message as read on multiple devices, the ability to send items from multiple devices, and having emails pushed to your device as they arrive.

In contrast, IMAP (Internet Message Access Protocol) is a more advanced protocol that allows the user to see all the folders on the mail server. With IMAP, the message does not remain on the local device, such as a computer, it remains on the server. This means that messages can be accessed across multiple devices, and users can search the content of mail for a specific string before downloading. IMAP is more flexible and complex than POP3, and it allows multiple clients to access the same mailbox while keeping the email messages available on the server for later access via webmail.

In summary, POP3 is an older protocol that only allows one-way email synchronization and is tied to a specific device, while IMAP is a more advanced protocol that allows multiple clients to access the same mailbox and keeps the email messages available on the server for later access via webmail.