Gyaan

SMTP, IMAP & POP3

beginner smtp imap pop3 email protocols

Email is older than the modern web, and it shows. We have three different protocols for what feels like one task.

In simple language: SMTP sends, IMAP and POP3 receive. Sender uses SMTP. The recipient’s mail client uses IMAP or POP3 to pull messages from their mailbox.

SMTP — Simple Mail Transfer Protocol

SMTP is the postman. It carries mail from our client to our outgoing mail server, and between mail servers across the internet.

  • Port 25 — server-to-server SMTP. Often blocked by ISPs to fight spam.
  • Port 587 — submission port for clients to send via their own server. Uses STARTTLS for encryption.
  • Port 465 — implicit TLS. Originally deprecated, now back in use.
# What an SMTP conversation looks like (simplified)
> HELO client.example.com
< 250 Hello
> MAIL FROM:<manish@example.com>
< 250 OK
> RCPT TO:<friend@gmail.com>
< 250 OK
> DATA
< 354 Send message
> Subject: Hi
>
> Hello friend.
> .
< 250 Message accepted
> QUIT

SMTP is push-based. The sending server contacts the receiving server (looked up via the recipient’s MX record) and pushes the message.

IMAP — Internet Message Access Protocol

IMAP keeps mail on the server. Our client (Gmail web, Apple Mail, Outlook) just shows what’s there.

  • Port 143 — plain.
  • Port 993 — IMAP over TLS.

Why IMAP wins for most people:

  • Same inbox on phone, laptop, web — read on one, marked as read everywhere.
  • Folders, flags, drafts all live on the server.
  • Server keeps the mail safe; client is just a viewer.

POP3 — Post Office Protocol v3

POP3 downloads mail to one device and (by default) deletes it from the server.

  • Port 110 — plain.
  • Port 995 — POP3 over TLS.

POP3 made sense when:

  • Storage on the server was expensive.
  • We had one PC and a slow dial-up line.
  • We didn’t need to read mail on multiple devices.

Today it’s mostly a fallback option. Most people should use IMAP.

IMAP vs POP3 — Quick Pick

  • Use IMAP if we read mail on more than one device.
  • Use IMAP if we want server-side folders and search.
  • Use POP3 only if we want to keep all mail locally on a single machine and the server has tiny storage.

How These Fit Together

                  ┌──────────────┐
   Manish writes  │   SMTP 587   │
   email ────────>│  (his mail   │
                  │   server)    │
                  └──────┬───────┘
                         │ SMTP 25
                         ▼
                  ┌──────────────┐
                  │  Recipient's │
                  │ mail server  │
                  └──────┬───────┘
                         │
            ┌────────────┴────────────┐
            ▼                         ▼
    IMAP 993 (sync)           POP3 995 (download)
            │                         │
            ▼                         ▼
       Friend's phone           Friend's old laptop

Modern Reality

Most people don’t touch these protocols directly anymore — Gmail, Outlook web, and similar use HTTPS-based APIs internally. But IMAP and SMTP are still what desktop mail clients (Thunderbird, Apple Mail) speak under the hood.

Interview Tip

Keep it short: SMTP = send, IMAP = sync from server, POP3 = download and delete. Remember the secure ports (587, 993, 995) because interviewers love port numbers.

References