Internet Message Access Protocol (IMAP) is a standard email protocol that stores email messages on a mail server, but allows the end user to view and manipulate the messages as though they were stored locally on the end user's computing device(s).
When you read an email message using IMAP, you aren't actually downloading or storing it on your computer; instead, you're reading it from the email service. As a result, you can check your email from different devices, anywhere in the world: your phone, a computer, a friend's computer.
IMAP only downloads a message when you click on it, and attachments aren't automatically downloaded.
This way you're able to check your messages a lot more quickly than POP.
Post Office Protocol version 3 (POP3) is a standard mail protocol used to receive emails from a remote server to a local email client.
POP3 allows you to download email messages on your local computer and read them even when you are offline.
POP3 works something like this. Your app connects to an email server, downloads all messages to your PC that have not been previously downloaded, and then deletes the original emails from the server.