Why POP3 Exists
POP3 — Post Office Protocol version 3, defined in RFC 1939 — was designed in 1988 for a world of dialup connections and single-user workstations. The model was simple: you dialled in briefly, downloaded all your mail to your local machine, and disconnected. The mail server was treated as a temporary holding area, not a permanent mailbox. Storage was expensive, servers were underpowered, and the idea of accessing the same inbox from multiple devices was not yet relevant.
POP3 executes that model faithfully. The client connects on port 110 (or 995 with TLS), sends USER and PASS commands, then lists and retrieves messages with LIST and RETR. After downloading each message, the client typically sends DELE to mark it for deletion on the server. When the session ends with QUIT, the server purges all messages marked for deletion. The client now has local copies; the server is empty and ready for the next batch of incoming mail.
How IMAP Changed the Model
IMAP — Internet Message Access Protocol — was designed from the outset for a different assumption: email lives on the server permanently, and clients are just views into that server-side mailbox. Mark Crispin developed IMAP in 1986 partly in response to POP3's limitations, and it became the dominant protocol as always-on internet connections and multi-device access became normal.
With IMAP, no message is downloaded and deleted by default. Instead, the client maintains a synchronised view of the server-side mailbox. Reading a message marks it as read on the server, so every other client — your phone, your laptop, your webmail — immediately sees the same read status. Moving a message to a folder on your desktop moves it in the server's folder structure, making it appear in that folder on every device. Flagging, starring, and deleting all operate on the canonical server-side state.
The Synchronisation Difference in Practice
The difference becomes concrete the moment you use a second device. With POP3 configured to download and delete: your laptop downloads Monday's messages. When you check email on your phone later, those messages are gone — the server no longer has them. Your phone sees only Tuesday's messages that arrived after the laptop's last session. There is no shared inbox; each device has an independent subset of your mail history.
With IMAP, both devices see the same inbox at all times. The server holds every message. Both clients display the current state. An email read on your phone is marked read when you open your laptop. A message deleted from webmail disappears from your IMAP client on the next sync. This coherence is why IMAP became universal the moment smartphones arrived.
IMAP IDLE: Near-Instant Delivery
Traditional email retrieval is polling — the client connects every few minutes and asks the server whether anything new has arrived. IMAP IDLE changes this: after the client opens a mailbox, it sends an IDLE command and the server holds the connection open. When a new message arrives, the server sends an EXISTS notification immediately, and the client fetches it without waiting for the next polling interval. The result is push-like email delivery that notifies you within seconds of a message arriving, using a single persistent TCP connection rather than repeated reconnections.
IMAP vs POP3: Feature Comparison
| Feature | IMAP | POP3 |
|---|---|---|
| Message storage | Stays on server permanently | Downloaded to client, deleted from server |
| Multi-device sync | Yes — all devices see the same state | No — each device gets an independent copy |
| Folder synchronisation | Yes — server-side folders synced everywhere | No — folders are local only |
| Read/unread sync | Yes | No |
| Standard port (TLS) | 993 (IMAPS) | 995 (POP3S) |
| Push notification | Yes — via IMAP IDLE | No — polling only |
| Server storage required | Yes — grows with mail history | Minimal — messages removed after download |
| Best for | Any multi-device setup | Single-device offline archive with low server storage |
When POP3 Still Makes Sense
POP3 is not entirely obsolete. It remains useful in a narrow set of scenarios. If you want a guaranteed local archive that exists independently of your email provider — one that survives a provider account closure or data loss — POP3 with "leave a copy on server" disabled gives you a definitive local copy. Some compliance and legal archiving workflows require downloading and storing email in a write-once archive, and POP3's download-and-delete model maps naturally to that pattern.
POP3 also requires less server infrastructure: no persistent connections, no IDLE support, no complex folder hierarchies. For embedded systems or appliances that need to collect email notifications from a mailbox without the overhead of IMAP, POP3's simplicity is an advantage. These are niche cases — for any general-purpose email setup with more than one device, IMAP is the right choice.
Security: Always Use TLS
Both IMAP and POP3 have TLS-encrypted variants: IMAPS on port 993 and POP3S on port 995. Configuring a mail client to connect on the unencrypted ports (143 for IMAP, 110 for POP3) without TLS sends your password and every email message in plaintext — readable by anyone who can observe the network between you and the server. Modern email providers require encrypted connections and may refuse unencrypted login attempts entirely. Always configure port 993 for IMAP and port 995 for POP3, with implicit TLS enabled.
Frequently Asked Questions
Does POP3 delete email from the server?
By default, yes. POP3 was designed to download messages to a local mail client and then remove them from the server. Most modern email clients offer a "leave a copy on server" option when configured for POP3, but this does not provide folder synchronisation — the copy on the server is just a backup, not a synced mailbox.
Can I use both IMAP and POP3 for the same account?
Technically yes, but mixing them on different devices creates confusion: the POP3 client downloads and may delete messages that your IMAP clients expect to remain on the server. For any account accessed from more than one device, use IMAP consistently on all devices to ensure a coherent, synchronised mailbox.
What port does IMAP use?
IMAP uses port 143 for unencrypted connections and port 993 for IMAP over TLS (IMAPS). Always use port 993 with TLS — configuring a mail client to connect on port 143 without TLS sends your password and all email contents in plaintext. Modern email providers default to port 993 with implicit TLS.
Why is IMAP better than POP3 for phones?
IMAP stores email on the server and syncs read/unread status, folders, flags, and deletions across every connected device. When you read a message on your phone, it is marked read on your laptop and webmail immediately. With POP3, each device downloads its own independent copy — reading on your phone has no effect on the copy your laptop downloaded.
Does IMAP use more storage than POP3?
IMAP uses more server storage because messages remain on the server permanently rather than being downloaded and deleted. This is why email providers impose mailbox size quotas. For users, it means your complete email history is preserved on the server and accessible from any device — but you must occasionally archive or delete messages to stay within your quota.
What happens to email if I switch from POP3 to IMAP?
Messages already downloaded by your POP3 client remain in that client's local storage — they are not automatically uploaded to the server. For a clean migration, manually drag-and-drop old messages from your local POP3 mailbox into your IMAP folders so they are uploaded to the server and accessible everywhere.