Email Protocols (SMTP, POP3, IMAP)

Email uses a combination of protocols: one for sending, one or more for receiving.

SMTP (Simple Mail Transfer Protocol)

Used for sending emails — from a client to a server (submission) and between servers (relay).

Port	Encryption	Use
25	None (plain)	Server-to-server relay
587	STARTTLS	Client submission (authenticated)
465	SSL/TLS	Client submission (deprecated but common)

SMTP is a push protocol — it pushes messages towards the recipient’s server.

POP3 (Post Office Protocol v3)

Used for receiving emails. Downloads messages from server to client, then typically deletes them from the server.

Pros	Cons
Low server storage usage	Not suitable for multiple devices
Works offline	If device is lost, emails are gone
Simple, fast	No server-side folder management

IMAP (Internet Message Access Protocol)

Used for receiving and synchronizing emails. Messages stay on the server.

Pros	Cons
Syncs across all devices	Requires server storage
Manages folders on server	Slightly more complex
Partial fetch (download only headers)	More bandwidth on initial sync

IMAP is the modern standard — it keeps the server as the source of truth.

Email Flow

Sender's MUA (Outlook) ──SMTP──→ Sender's MTA (Gmail)
                                       │
                                    SMTP
                                       │
                               Recipient's MDA (mailbox)
                                       │
                              POP3/IMAP│
                                       │
                              Recipient's MUA (Outlook)

• MUA = Mail User Agent (the email client)
• MTA = Mail Transfer Agent (server that relays email)
• MDA = Mail Delivery Agent (stores email in mailbox)