110 POP3 TCP Flagship
Reference for TCP port 110, commonly associated with POP3.
What it's for
Port 110 is the default port for POP3 (Post Office Protocol version 3), one of the two dominant protocols (alongside IMAP) for retrieving email from a mail server to a client. POP3's defining behavior is simple: it downloads messages to the client and, by default, deletes them from the server afterward — a "download and remove" model built for an era when people checked email from a single device.
That single-device assumption is why POP3 has faded in popularity as multi-device access (phone, laptop, webmail) became the norm — most modern setups use IMAP instead, which keeps mail synced on the server. POP3 still shows up in legacy mail clients, some minimal embedded systems, and configurations where someone deliberately wants a local-only archive.
Protocol basics
- Protocol: TCP, for reliable retrieval of message data.
- Plaintext by default. Standard POP3 on port 110 sends credentials and message content unencrypted — the encrypted variant runs on port 995 instead (POP3S).
- Simple command set. POP3's command vocabulary is intentionally minimal: authenticate, list messages, retrieve a message, delete a message, quit — far simpler than IMAP's folder/flag/partial-fetch model.
- "Leave a copy" is a client-side setting, not protocol default. Most modern clients configure POP3 to leave messages on the server for some retention period, but the base protocol's original behavior is delete-on-download.
Security considerations
- Never use plain POP3 (port 110) over an untrusted network. Since credentials and message bodies travel unencrypted, use POP3S (port 995) or, better, migrate to IMAP over TLS entirely.
- Delete-by-default behavior is a data-loss risk if misconfigured. A client set to "delete from server after download" with no local backup strategy can result in permanent mail loss if the device is lost or the mailbox isn't backed up.
- Rate-limit and monitor authentication attempts on any POP3 server you operate — like most login-based services, it's a target for credential-stuffing attempts.
Troubleshooting
Authentication fails despite correct credentials:
- Check whether the mail provider requires an app-specific password (common with providers that have 2FA enabled) rather than your normal account password
- Confirm you're using the correct port (110 for plaintext, sometimes with STARTTLS, versus 995 for implicit TLS)
Messages disappear from other devices after checking mail:
- Classic POP3 behavior — the client that downloaded the mail removed it from the server. Check the "leave messages on server" setting in whichever client checked first
Connection refused:
- POP3 service isn't running or enabled on the mail server, or a firewall is blocking port 110
Emails download but attachments are missing/truncated:
- Usually a client-side configuration or size-limit issue rather than a POP3 protocol problem — POP3 retrieves the full message including attachments by default
Comparison / FAQ
| Port | Protocol | Key difference from 110 (POP3) |
|---|---|---|
| 995 (POP3S) | TCP | Same protocol, wrapped in implicit TLS encryption |
| 143 (IMAP) | TCP | Keeps mail synchronized on the server across multiple devices, rather than downloading and removing it |
| 25 (SMTP) | TCP | Handles sending mail; POP3 only handles retrieval, the opposite direction |
Should I use POP3 or IMAP for a new email setup?
IMAP is almost always the better choice today, since it keeps your mailbox synchronized across every device you check mail from. POP3 mainly makes sense if you specifically want a single local archive and don't need multi-device access.
Is POP3 insecure by default?
Plain POP3 on port 110 is unencrypted, meaning credentials and messages are visible to anyone observing the network. POP3S on port 995 solves this by wrapping the same protocol in TLS.
Why did my emails disappear from my phone after checking them on my laptop?
This is expected POP3 behavior if the laptop's client is configured to delete messages from the server after downloading. Enabling "leave a copy on the server" (and setting a retention period) in that client fixes the issue going forward.
Can I still use POP3 with modern providers like Gmail?
Most major providers still support POP3 access, though it's often disabled by default and needs to be explicitly enabled in account settings, since IMAP is the recommended default for multi-device use.