IMAP, an Internet Standard Protocol
IMAP is an Internet Message Access Protocol which was originally developed at Stanford University in 1986. IMAP is a widespread method of accessing bulletin board or electronic mail messages stored on a mail server. IMAP was designed with the purpose of providing complete management of an email box by multiple email clients so clients usually leave messages on the server until the user finally deletes them.
IMAP enables a "client" email program to get access to remote message storage as if it was local. Users can check email stored on IMAP server from any device wherever they are a workstation in the office, a desktop at home, and a laptop or a phone while traveling. There is no need to transfer files and messages back and forth between these devices.
IMAP's key feature to provide access to saved and new messages from more than one device became very important today since the usage of electronic messaging, computers, and mobile devices increased substantially.
When a user reads an email message using IMAP, he isn't really downloading or storing this message on his computer, instead, he is reading the message from the email service where he has an account. As a result, users can check their email from different devices such as phones, desktops, tablets from any point in the world.
IMAP will only download a message when a user clicks on it, and the attachments aren't downloaded automatically. This way, users have the possibility to check their messages much more quickly than POP, a widely used Post Office Protocol created to support offline message access when messages are downloaded from a mail server onto user’s PC. The sent email is deleted from the email service at once and is stored locally on the user’s computer.
Practically all modern e-mail clients and servers support IMAP.
You might be interested in SFTP Data Transmission Protocol.