SMTP (Simple Mail Transfer Protocol) is used to send emails from a client to a mail server, and between mail servers. Port 25 (or 587 for submission). SMTP only handles outgoing mail — a separate protocol is needed to receive.
When you press "Send" in Gmail, your device uses SMTP to forward the email to Google's mail server
The server then uses SMTP to forward to the recipient's mail server
The recipient retrieves it using POP3 or IMAP
POP3 vs IMAP — Receiving Email
Two Ways to Read Your Inbox
POP3 (Post Office Protocol v3): downloads emails from the server to your device and usually deletes them from the server. Good for one device only. Port 110. Simple, works offline.
IMAP (Internet Message Access Protocol): emails stay on the server and are synchronised across all devices. Changes made on one device appear on all others. Port 143. Ideal for multi-device access.
POP3 vs IMAP Comparison
Choosing the Right Protocol
Storage: POP3 saves to local device (less server storage); IMAP stores on server (needs more server space)
Multi-device: POP3 → email only on one device; IMAP → same inbox on phone, laptop, tablet
Offline access: POP3 works fully offline after download; IMAP needs server connection for full functionality
Modern preference: IMAP is now the default for most email clients (Gmail, Outlook)
Exam Practice
Have a go at this question
Edexcel-style question
A user checks their emails on both their phone and their laptop. Explain why IMAP is more suitable than POP3 for this user.
3 marks
IMAP stores emails on the server [1], so the same inbox is synchronised across all devices [1]. If an email is read on the phone, it is also marked as read on the laptop, unlike POP3 which downloads to one device and removes from the server [1].
Key Takeaways
What to Remember
SMTP: sends email (outgoing); port 25/587
POP3: downloads and removes from server; one device; port 110
IMAP: syncs with server; all devices see same inbox; port 143
IMAP preferred for multi-device; POP3 better if limited server storage