network – Does mail server send retrieved messages over HTTP? How?


Does an email server send retrieved messages over HTTP? How? (I know POP3 uses RETR command and IMAP uses FETCH command, but what does HTTP use?)

Simply searching online says email servers exclusively use SMTP and usually POP3 or IMAP. My textbook, however, says that email servers can use HTTP to send from an email server to web client.

Image and quote source: Computer Networking: A Top-Down Approach, a university networking course textbook

With this service, the user agent is an ordinary Web browser, and the user communicates with its remote mailbox via HTTP. When a recipient, such as Bob, wants to access a message in his mailbox, the e-mail message is sent from Bob’s mail server to Bob’s browser using the HTTP protocol rather than the POP3 or IMAP protocol. When a sender, such as Alice, wants to send an e-mail message, the e-mail message is sent from her browser to her mail server over HTTP rather than over SMTP.

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