How do I mark an email as read or unread?
How do I download only the unread emails from a POP3 server?
These are common questions. The trouble is that POP3 servers do not maintain read/unread status information, so it is impossible to query a POP3 server for unread emails, or to mark an email as read/unread. (IMAP servers are different, and do support and maintain the read/unread status of each email.)
Programs such as Outlook, Thunderbird, etc. save the read/unread information in their local database on your computer’s filesystem. The intended usage model for POP3 is to download email from the server and onto your local computer (or database) where the mail is managed locally. POP3 mailboxes are intended to serve as maildrops. The intended usage model of IMAP is the opposite: email is kept and maintained on the server. Because of this, the IMAP protocol is more feature rich and includes functionality such as read/unread status.