SMTP, IMAP, POP3, which are TCP protocols, all give users the ability to send and receive their emails. SMTP sends emails to clients, while the other two protocols collect emails from clients.
If the person sending as well as the person receiving the email have opposite providers, 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 the servers.
POP (POST Office Protocol) was originally made to download copies of emails for offline use. The first version of POP (RFC 918) was in 1984, with POP2 (RFC 937) created in 1985. POP3 (RFC 1939) is the latest version, first introduced in 1988, and provides protection against unauthorized access to a user’s e-mail. POP3 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 gone, the user cannot log on from elsewhere and see those same emails, they can only be viewed in the first mail client because that is where they were downloaded to and saved.