What is POP3 (Post Office Protocol version 3)?
Learn what POP3 is, how it downloads and removes mail from the server, its ports, and how it differs from IMAP.
Expected Interview Answer
POP3 (Post Office Protocol version 3) is an application-layer protocol for retrieving email that traditionally downloads messages from the mail server to a single client and, by default, deletes them from the server afterward, unlike IMAP which keeps everything synchronized server-side.
A POP3 client connects to the mail server, usually on port 110 or the encrypted port 995, authenticates, and pulls down all waiting messages in the mailbox to local storage. Once downloaded, the default behavior is to remove the copy from the server, meaning the mail now exists primarily on the one device that fetched it, which makes POP3 a poor fit for anyone checking mail from multiple devices since a phone that already downloaded a message would leave a laptop with nothing left to fetch. Most modern clients offer a “leave a copy on server” option to soften this, but that still does not give the live, two-way folder synchronization IMAP provides. POP3 remains useful for simple, single-device setups, low-bandwidth environments, or archiving mail permanently offline.
- Simple protocol for pulling mail down to a single device
- Reduces server storage needs since mail can be removed after download
- Works well for low-bandwidth or offline-first mail setups
- Straightforward text-based command set for authentication and retrieval
AI Mentor Explanation
POP3 is like a courier who visits the clubhouse mailbox once, physically takes every letter waiting there, and hands them all to one specific coach, leaving the clubhouse mailbox empty afterward. If a second coach checks that same mailbox later, there is nothing left because the first courier already emptied it. This mirrors how POP3 downloads mail to one client and, by default, removes it from the server, unlike a system where every coach can see the same shared, persistent mailbox.
Step-by-Step Explanation
Step 1
Connect and authenticate
The client connects to the POP3 server, usually on port 995 (TLS), and logs in with credentials.
Step 2
Retrieve message list
The client requests a list of waiting messages and their sizes.
Step 3
Download messages
The client pulls each full message down to local storage on that one device.
Step 4
Delete from server
By default, downloaded messages are removed from the server unless “leave a copy” is enabled.
What Interviewer Expects
- Knows POP3 downloads mail and removes it from the server by default
- Can name the standard ports (110 plain, 995 with TLS)
- Understands why POP3 is a poor fit for multi-device access
- Can contrast POP3 with IMAP's server-side synchronization model
Common Mistakes
- Assuming POP3 keeps mail synchronized across multiple devices like IMAP
- Confusing POP3's ports 110/995 with IMAP's 143/993
- Not knowing “leave a copy on server” is an option, not the default behavior
- Thinking POP3 handles sending mail rather than only retrieval
Best Answer (HR Friendly)
“POP3 is an older way of checking email where the client downloads every message straight to one device and, by default, removes it from the server afterward — like collecting all your mail from a P.O. box and taking it home, leaving the box empty. It works fine if you only ever check email from one device, but it falls apart if you also want the same inbox to show up on your phone, which is why most people use IMAP today instead.”
Code Example
import poplib
conn = poplib.POP3_SSL("pop.example.com", 995)
conn.user("[email protected]")
conn.pass_("app_password")
count, total_size = conn.stat()
print(f"{count} messages waiting, {total_size} bytes total")
for i in range(1, count + 1):
response, lines, octets = conn.retr(i)
print(f"Downloaded message {i}: {octets} bytes")
# conn.dele(i) # marks message for deletion from the server
conn.quit()Follow-up Questions
- Why is POP3 a poor fit for checking email on multiple devices?
- What does the “leave a copy on server” setting actually change?
- How do POP3's ports 110 and 995 differ?
- When might POP3 still be a reasonable choice today?
MCQ Practice
1. What is the default behavior of POP3 after downloading a message?
By default, POP3 removes the message from the server once it has been downloaded to the client.
2. Which port does POP3 use with TLS encryption?
Port 995 is the standard POP3-over-TLS port; port 110 is the unencrypted default.
3. Why is POP3 generally unsuitable for multi-device email access?
Since mail is usually deleted from the server after download, other devices see nothing left to retrieve.
Flash Cards
What is POP3? — A protocol that downloads email to a client and, by default, removes it from the server.
POP3 secure port? — Port 995 (TLS); port 110 is the unencrypted default.
POP3 vs IMAP? — POP3 downloads and typically removes mail; IMAP keeps mail synchronized on the server.
Is POP3 good for multiple devices? — No — mail pulled to one device is usually gone from the server for others.