What is port 110 used for?

Port 110 is commonly used for the Post Office Protocol version 3 (POP3). POP3 is an application-layer Internet standard protocol that allows email clients to retrieve email from a remote mail server over a TCP/IP connection.

What is port 110 used for?

How POP3 and port 110 enable access to email

POP3 is designed to enable users to download email messages from a server to a local client computer for storage and handling. Here is an overview of how it works:

  • Email clients connect to a mail server on port 110 using POP3
  • This allows the email client to access and download new messages to the user’s device
  • Messages are stored locally on the device once downloaded via POP3
  • The mail server retains a copy of the emails until the client finishes downloading them

So in summary, port 110 allows email clients to access and retrieve emails from a remote mail server via POP3 protocol. This permits users to download messages to their local device for offline access and storage.

Common email clients that use port 110

Many popular email clients and services utilize port 110 POP3 functionality to enable access to email, including:

  • Apple Mail
  • Microsoft Outlook
  • Windows Mail
  • The Bat!
  • Mozilla Thunderbird
  • Mac Mail
  • Yahoo Mail
  • Gmail (with POP3 enabled)
  • Hotmail (with POP3 enabled)
  • AOL mail (with POP3 enabled)

Any end-user email program that offers POP3 connectivity will connect via port 110 to download and access messages stored on a server.

Web-based email that is accessed via browser generally uses other protocols such as IMAP rather than POP3, but major webmail providers often allow users to enable POP3 accessibility as well.

Benefits of POP3 protocol and port 110

There are a few core benefits provided by the POP3 protocol that make port 110 valuable:

  • Offline access – Messages get downloaded locally so users can read email without an active internet connection.
  • Portability – Email is accessible across multiple devices after being downloaded via POP3.
  • Backups – Downloading mail to a local client allows users to backup messages.
  • Storage – Storing messages locally often gave users more mailbox storage space historically.

POP3 and port 110 cater primarily to users that want to be able to access their email without needing ongoing internet connectivity. It permits offline functionality and management capabilities email providers historically lacked.

Privacy, security, and port 110 access

There are some vital security considerations associated with enabling port 110 POP3 access:

  • Encryption – Traffic sent over port 110 for POP3 is often unencrypted by default. Using SSL security for “POP3S” is recommended to encrypt connections.
  • Authentication – Most servers require username and password authentication before permitting access to port 110. This helps prevent unauthorized access.
  • Firewall rules – Admins commonly restrict port 110 traffic via firewall policies to prevent unwanted network connectivity on that port. Specific IP addresses may need to be whitelisted.
  • Access controls – Mail administrators should carefully evaluate and confirm what level of access to enable for POP3 to balance security needs with email accessibility requirements.

As with any port that enables external network access to data, proper security protections should be in place when allowing connectivity via port 110. Restricting access and requiring authentication helps keep port 110 secure.

Differences between POP3 on port 110 vs IMAP on port 143

IMAP and POP3 have some core differences in how they enable email access that are helpful to be aware of:

Uses port 110 to download messages to a local device Uses port 143 to access messages directly on the mail server
Doesn’t synchronize read/unread status across devices Syncs message state across multiple connected devices
Requires downloading all new messages to local device first Allows users to access mail on server without needing to download locally first
Deletes messages from server after local download completes Leaves messages on the server by default after user access
Offline functionality – messages persist on local device without connectivity Requires connectivity to mail server to access messages
Historically offered limited server storage so downloading messages was necessary More modern solution as IMAP servers include sufficient storage to retain all messages

In summary, POP3 uses port 110 to facilitate one-way downloading of messages to a client device while IMAP uses port 143 to enable two-way synchronized access directly to mail stored on a server.

TCP vs UDP for port 110

The POP3 protocol communicates over port 110 using TCP as the core transport mechanism:

  • TCP ensures reliable delivery of all network packets in the proper order
  • TCP handles error checking and retransmissions if issues occur
  • TCP establishes a connection-oriented session between client and server for consistent data transfer
  • TCP has mechanisms to adapt to network conditions with flow control, congestion avoidance, etc.

This reliability and error handling means TCP is well suited for an application-level email access protocol like POP3. Proper sequence and delivery of packets is essential to successfully transfer complete message data to an email client.

UDP does not have these transmission assurances and error handling capabilities. So UDP would likely encounter issues with disrupted email transfers.

For exchanging consistent, complete application information streams like email access, TCP over port 110 is standard.

Can other protocols utilize port 110?

While port 110 and POP3 are commonly associated, in some rare cases port 110 can be used for other protocols, though this is not typical or recommended. SMTP is sometimes seen using port 110 improperly, often unintentionally due to a configuration error.

Here are a few examples of other limited situations where port 110 may be leveraged differently:

  • SMTP misconfiguration – SMTP servers accidentally get assigned to port 110 instead of standard port 25 traffic
  • SQL Server resolution – Default SQL Server instance traffic resolution when UDP port 1434 is unavailable
  • ICQ instant messaging – Some old versions of the ICQ client messenger used port 110
  • Proxy and VPN services – Within an isolated network, devices may be configured to tunnel non-standard protocols over port 110

However, officially and by convention, port 110 is defined for POP3 traffic and should not be used for other purposes, especially externally facing across firewall boundaries.

So while other protocols may in special cases utilize port 110, this is not advisable or formally correct. POP3 over port 110 should be maintained exclusively according to official IANA port assignments.

Key Takeaways on Port 110

  • Port 110 is formally designated for POP3 email access traffic
  • POP3 allows email clients to connect and download messages from mail servers
  • Common mail clients like Outlook leverage port 110 POP3
  • Benefits include offline message access, backups, storage, and portability
  • Security protections like encryption and firewall rules should wrap port 110
  • IMAP differs in synchronizing server mail access instead of one-way POP3 downloads
  • TCP enables reliable delivery of email message data to clients
  • Only POP3 should formally use port 110, though exceptions sometimes incorrectly occur


In summary, port 110 plays an important role in permitting email client applications to access mail stored remotely on servers. The POP3 protocol provides offline message functionality, portability across devices, potential for backups, and storage capacity extension.

Both individual users and corporate environments rely on port 110 availability to leverage POP3 benefits – accessing mail from multiple locations, enabling mobility, facilitating backups, and providing continuity through offline mailbox availability when Internet connectivity is disrupted.

It fills an important niche for email access alongside more real-time server-based protocols like IMAP. Security teams need visibility into port 110 traffic to detect potential issues, restrict access appropriately, and ensure POP3 connections are encrypted.

As an officially designated port for POP3, port 110 should serve that exclusive purpose based on standards and best practices. Proper configuration of mail clients, servers, firewalls and other infrastructure should maintain port 110 for POP3 to avoid disruptions.

With protections in place, port 110 delivers immense value in extending email flexibility through POP3 protocol availability in modern computing.

Frequently Asked Questions 

  1. Why do email clients use port 110?
    Port 110 is specifically designated for POP3, which allows email clients to connect to servers to retrieve email messages. So all POP3 client traffic occurs over port 110.
  1. Is port 110 TCP or UDP?
    Port 110 uses TCP as the protocol since it provides reliable, ordered data transfer critical for proper email transfers.
  1. Is port 110 secure?
    By itself, no. Unencrypted POP3 connections over port 110 should be considered unsecure. POP3S with SSL encryption should be implemented to secure port 110 traffic.
  1. Can port 110 be blocked by a firewall?
    Yes, administrators often restrict or block access to port 110 via firewall rules as a security precaution. Clients would then be unable to access POP3 mail.
  1. What is a POP3 port number?
    POP3 uses port 110, and the secure POP3S uses port 995, on top of native SSL encryption. So POP3 officially leverages port 110.
  1. What is the difference between IMAP and POP3?
    IMAP uses port 143 to directly access mail on the server, while POP3 uses port 110 to just download messages locally. IMAP also synchronizes state across devices, unlike POP3 behavior.
  1. Is IMAP or POP3 better?
    It depends on the use case. IMAP facilitates accessing the same mail across multiple clients. POP3 enables offline mail functionality as messages get downloaded to individual devices separately.
  1. Can you tell the difference between POP3 and IMAP traffic?
    Yes, Wireshark and other packet analyzers can identify POP3 on port 110 vs IMAP traffic on port 143 based on the way the protocols communicate differently.
  1. What uses port 110?
    By standards and conventions, port 110 should only be used for POP3 email access traffic, though exceptions exist where other protocols use it incorrectly. Formally it is for POP3 only though.
  1. Is port 110 TCP or UDP?
    Port 110 almost always leverages TCP as the protocol since ordered, reliable data delivery is essential for POP3 email transfers between clients and servers.
  1. Why are POP3 connections not encrypted?
    Historically they often were not, but modern clients should enable POP3S connections over TLS encryption like SSL to secure port 110 traffic as needed.
  1. What app uses port 110?
    Email clients that support POP3 functionality communicate over port 110 to fetch messages from mail servers into local storage.
  1. Can port 110 be used for anything else?
    In some irregular scenarios port 110 gets used for protocols like SMTP or VPN connections, typically incorrectly due to configuration errors. Formally and ideally it is reserved for POP3 exclusively.
  1. Is port 110 inbound or outbound?
    From the client’s perspective, it would establish an outbound connection over port 110 to the server to download POP3 mail. Inbound firewall rules must also permit the response traffic.
  1. Should port 110 be open or closed?
    For POP3 functionality, port 110 needs to accept connections and remain open, restricting sources appropriately. Local firewall rules would limit traffic to only approved POP3 servers.
  1. Is port 110 open by default?
    It varies by OS and network. Consumer routers may block port 110, while data center firewalls tend to have it open by default since blocking ports can impact services.
  1. Can I change port 110?
    Changing the numeric assignment would go against conventions, but altering the protocol it carries via tunneling or other means is possible, though not compliant.
  1. Is port 110 user configurable?
    No. Port 110 is standardized to POP3 formally. However, users can disable POP3 functionality in their mail client or firewall policies. But the port itself can’t be modified directly.
  1. Why choose port 110 for POP3?
    Likely because it was the next standardized available port at the time after well-known ports like SMTP port 25, FTP port 21, Telnet port 23 etc.
  1. What port does Outlook use for POP3?
    By default Microsoft Outlook will attempt to use configured POP3 servers via port 110 for downloading email locally, unless IMAP functionality is enabled instead.

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