POP (Post Office Protocol) a protocol used to retrieve email from a mail server. POP has been developed through several versions, with version 3 (POP3) being the last standard in common use before largely being made obsolete by the more advanced IMAP as well as webmail.
Incoming messages are stored at a POP server until the user logs in using an email client and downloads the messages to their computer. Since this is the way POP normally functions you will only be able to store emails from the POP server on the device it is accessed by and will not be able to have your emails on multiple devices.
IMAP (Internet Message Access Protocol) is an Internet protocol that allows an email client to access email on a remote mail server. An IMAP server typically listens on port 143. IMAP over SSL is assigned port number 993.
IMAP supports both on-line and off-line modes of operation. Email clients using IMAP generally leave messages on the server until the user explicitly deletes them. This and other characteristics of IMAP operation allow multiple clients to manage the same mailbox while maintaining "sync" regardless of which client, device or application is used to view, send or receive email.
Incoming email messages are sent to an email server that stores messages in the recipient's email box. The user retrieves the messages with an email client that uses one of a number of email retrieval protocols. Most clients support the Internet standard protocols, SMTP for sending e-mail and POP and IMAP for retrieving email, allowing interoperability with other servers and clients.
What should I use?
Both POP (Post Office Protocol) and IMAP (Internet Message access protocol) allow people to get access to their email from a remote server; however, that is where most similarities end. POP simply downloads email to your computer, and usually deletes the email from the remote server. The problems arise if you have more than one device where you read your mail (desktop, laptop, tablet or phone). Logging into each device, you will see lots of unread emails with no indication of which you deleted, read, flagged or filed. Any folders you created and organize on one device won't be replicated on the other devices
IMAP allows users to store their email on remote servers. This two-way protocol also allows the user to synchronize their email among multiple devices, which is extremely important today, when most people have at least two devices - their laptop and smartphone.
We recommend backing up your mailbox before switching to IMAP.