Mail servers can be divided into two categories: incoming mail servers and outgoing mail servers.
Incoming mail server
An incoming mail server stores mail and sends it to a user's inbox. Post Office Protocol 3 (POP3) and Internet Message Access Protocol (IMAP) are the two main types of incoming mail servers.
POP3, for example, downloads email from a server and stores incoming email messages on a single device until the user opens the email client. Once the user downloads the email, it is automatically deleted from the server, unless the "keep mail on server" setting is enabled. Many internet service providers offer their users POP3 email accounts, as they are more space efficient.
IMAP servers enable users to preview, delete and organize emails before transferring them to multiple devices from the email server. Copies of emails are left on the server until the user deletes them.
Outgoing mail server
An outgoing mail server operates by having a user's machine communicate with Simple Mail Transfer Protocol (SMTP), which handles the email delivery process. SMTP servers work with other types of mail servers, namely POP3 or IMAP, to send emails from email clients.
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