SMTP, IMAP, POP3, 3 TCP protocols, all give users the ability to send and receive their emails. SMTP sends emails out to clients, while the other two protocols retrieve emails from clients.
If the sender as well as the recipient of an email have opposite providers for their email, the capacity to send messages between servers is needed. SMTP (Simple Mail Transfer Protocol) is the protocol for sending out email from an email client like Microsoft Outlook to an email server. It’s also used to forward email messages between mail servers.
POP was originally created to download copies of emails for offline use. The first version of POP was created in 1984, with POP2 created in 1985. POP3 is the latest version of the Post Office Protocol (POP), first introduced in 1988, and is used to collect email messages from a mail server that then go to a mail client. POP3 also deletes copies of the messages stored on the server, and then disconnects. Once the emails are removed from the server, the user cannot log on from any other device or client and see those emails, they can only be viewed in the original mail client because that is where they are downloaded to.