The server knows when you've read a message, deleted it, replied to it, put it into a folder, and so on. However, you can have multiple IMAP connections to the server showing you that copy. Even if you set up POP3 so it doesn’t delete all messages from the server, it will mark the server copy as “seen” as soon as it downloads them.
POP3 usually also immediately deletes all messages from our servers unless you take extra steps: since your computer has the only copy of the message, you need to rely on your own backups. Subsequent changes to that message on your computer don't affect the server or any other program or device reading the same mailbox. POP3 downloads copies of each message from the Inbox (only) of the server to your computer, which then keeps its own separate copy of each message.
Our servers fully support either method - it’s up to you, although we recommend IMAP in most cases. All modern email programs offer IMAP as well as the older POP3 standard. IMAP is a method of connecting to a server and reading incoming mail.
