IMAP
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SMTP delivers your email to your provider's server, but it does not show it to you. The protocol your phone or mail app uses to reach into your mailbox on that server and read your messages is IMAP. If SMTP is the mail truck dropping letters at the post office, IMAP is you (or your app) going to your numbered box at the post office and browsing what is there — except the letters stay filed at the office, and you just look at copies.
Concretely, your mail client connects to the IMAP server, logs in, and can then list folders, fetch message headers (to show a subject list without downloading everything), download a full message only when you open it, mark messages read or flagged, move them between folders, and search — all while the canonical copy stays on the server. Because the server holds the master copy, every device you own sees the same mailbox: read a message on your laptop and it shows as read on your phone, because both are just views onto the same server-side state. This is what makes modern multi-device email feel seamless.
It helps to contrast IMAP with POP3, the older retrieval protocol. POP3's classic model is download-and-delete: it pulls messages down to one device and removes them from the server, which suited the era of a single desktop and a slow link but breaks badly when you have a phone, a laptop, and a tablet. IMAP keeps mail server-side and syncs state, which is why it dominates today. Note that both IMAP and POP3 are retrieval protocols — neither sends mail; sending always goes through SMTP. The three together (SMTP out, IMAP/POP3 in) are how email actually works.
On the train you open mail on your phone, read a message, and archive it. That afternoon you open your laptop: the same message is already read and already in the archive folder. Neither device "owns" the mail — the IMAP server holds it, and both devices simply synced their view of that one server-side mailbox.
IMAP keeps the master copy on the server and syncs state, so every device sees one consistent mailbox.
IMAP and POP3 only retrieve mail — they never send it. Sending always uses SMTP. A common confusion is thinking IMAP "sends" replies; in fact your reply is composed locally and pushed out over SMTP.