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Email server and its protocols
Short answer
Email system comprises user agents (compose/read), mail servers with MTAs, mailboxes, and queues, and mail access agents. SMTP (port 25/587) sends mail between servers and from user agents; POP3 (110) or IMAP (143) retrieves mail; HTTP/HTTPS enables webmail. Mail flows: UA→SMTP→sender's server→SMTP→recipient's server→POP3/IMAP→UA.
What do you mean by an email server (components)? Which protocols are used in sending and receiving email? Illustrate with necessary figure.
Email server and its components
An email system lets users send messages that are stored and forwarded through servers. It has three main components:
- User agent (UA): the program used to compose, read and reply to mail (Outlook, Thunderbird, Gmail app).
- Mail server: contains the mailbox (stores incoming messages of each user), a message queue (outgoing mail waiting to be sent) and the MTA (Mail Transfer Agent) that talks SMTP to other servers.
- Mail access agent / protocol: retrieves mail from the server (POP3, IMAP, HTTP).
Alice UA --SMTP--> [Alice's mail server] --SMTP--> [Bob's mail server]
(MTA, queue) (MTA, mailbox)
|
POP3/IMAP/HTTP
v
Bob UA
Protocols
| Protocol | Role | Port |
|---|---|---|
| SMTP | Sending: UA to server and server to server (push) | 25, 587 |
| POP3 | Retrieval: downloads mail from server to the client | 110 |
| IMAP4 | Retrieval: mail stays on the server, folders managed there | 143 |
| HTTP/HTTPS | Webmail access | 80/443 |
| MIME | Extension of message format for non-ASCII and attachments | - |
Process
- Alice composes mail and her UA sends it to her mail server with SMTP.
- Her server looks up the MX record of Bob's domain in DNS, opens a TCP connection to Bob's mail server and sends the message using SMTP.
- The message is stored in Bob's mailbox.
- Bob's UA retrieves it using POP3 or IMAP.
✓ Answer checked by Claude Opus 5.5 (high effort)
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