Electronic Mail

Computer Network & Network Security System — Application Layer, NEC licence examination syllabus (Nepal Engineering Council).

Electronic Mail: the surprisingly old system still running your inbox

Email's core protocols predate the World Wide Web itself.

📧 The Three Email Protocols

SMTPSimple Mail Transfer Protocol — used to SEND email, both from your device to your mail server, and between mail servers.

POP3Post Office Protocol — downloads email onto your device and typically removes it from the server. Older, simple, but bad for checking mail from multiple devices.

IMAPInternet Message Access Protocol — keeps email on the server and syncs its state (read/unread, folders) across all your devices. This is what your phone and laptop both use to show the same inbox.

💡 One-liner: "Why does IMAP suit modern multi-device usage better than POP3?" → IMAP keeps mail synced on the server, so reading an email on your phone shows it as read on your laptop too — POP3 has no such awareness across devices.

One protocol pushes, two pull

The three protocols are easier to hold as a split than as a list, and the split explains the whole delivery path.

Sender's client ──SMTP──▶ Sender's server ──SMTP──▶ Recipient's server │ Recipient's client ◀──┘ POP3 or IMAP

SMTP is a push protocol: it moves mail toward its destination, which is why it appears twice — once from the sender's client and again between servers. POP3 and IMAP are pull protocols: they retrieve mail already sitting in a mailbox. That is why you cannot send with IMAP and cannot read with SMTP, a distinction questions test directly.

💡 It also explains why webmail needs no retrieval protocol at all. The mailbox is on the provider's server and the browser reads it over HTTPS, so POP3 and IMAP never enter the picture — though SMTP is still doing the delivery between servers behind the scenes.

Choosing between POP3 and IMAP

📥 Where the mail lives

POP3Downloads and typically deletes from the server. Mail lives on one device, uses no server storage, and works offline. Lose the device and the mail is gone.

IMAPMail stays on the server and clients sync state — read status, folders, drafts. Any number of devices show the same mailbox, at the cost of server storage and needing a connection for full access.

The deciding question is where the authoritative copy lives. POP3 says the device; IMAP says the server. Everything else — multi-device sync, offline behaviour, storage cost, backup responsibility — follows from that one answer.

What SMTP does not do

SMTP was designed for a small, trusted network and carries no authentication of the sender's address. The From field is simply text the sending system supplies, which is why forging it is trivial and why email spoofing remains possible decades later.

The additions that address it — SPF, DKIM and DMARC — all work around SMTP rather than changing it, by publishing in DNS which servers may send for a domain and signing outbound messages. The original protocol is untouched, because too much of the world depends on it to alter it.

💡 SMTP also has no confidentiality of its own. TLS can protect a message hop by hop, but each server decrypts it before forwarding — so end-to-end secrecy needs PGP or S/MIME, which encrypt the content itself rather than the connection carrying it.

Syllabus points

Email architecture
SMTP, POP3, IMAP

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