What is Port 143?

by David Brown | Published on June 27, 2025

Quick Definition: Port 143 is used for Internet Message Access Protocol (IMAP), which enables remote email management. Emails are stored on the server, allowing multiple devices to sync with the same inbox. However, IMAP is not secure by default and requires a more secure implementation.

Email management has two approaches: you can store the email messages on the server or on the client machine. There are advantages to storing your email on the server, but like any protocol, security should be a primary consideration. In this article, we take a closer look at port 143, a Transmission Control Protocol (TCP) port involved in email management.

What is Port 143?

Port 143 is the default port for the Internet Message Access Protocol (IMAP). Its primary purpose is to retrieve emails from a remote server. One of the key features of IMAP is the ability to synchronize an email mailbox across multiple devices. IMAP on port 143 enables users to read, organize, and delete emails without downloading them to their local device.

What is Port 143 Used For?

Port 143 enables a form of remote email management in which all the emails are stored on the server, rather than the client machine. Users can organize and delete emails on the server. 

Unlike Post Office Protocol 3 (POP3), IMAP allows multiple devices to sync with the same inbox.

Ever wonder what TCP ports are and why they’re important? Check out this CBT Nuggets article to find out more.

This netstat -aon output shows currently listening TCP ports on the local machine. Ports like 135, 445, and 7680 are standard Windows services. Notice in the screenshot below that port 143 does not appear, meaning no IMAP service is actively listening. The sequence of ports in the 496xx range is dynamic, not manually assigned.

Common Applications of Port 143

IMAP is commonly used by email clients like Microsoft Outlook, Netscape, and Eudora to retrieve emails from remote servers. Other apps like Mozilla Thunderbird and Apple Mail also use port 143 to access and manage email messages on a remote server.

Port 143 vs. Port 993

The real problem with IMAP on port 143 is that it’s not secure. Data is transmitted in plaintext, making it vulnerable to interception and eavesdropping. Port 993, however, is associated with a secure version of IMAP called IMAP over SSL/TLS (IMAPS). There are ways to make IMAP on port 143 secure using STARTTLS, but port 993 uses encryption by default. 

Below is a comparison of the two ports/protocols. iMAP allows email messages to remain on the server, enabling real-time syncing across devices. Port 143 supports IMAP without encryption, while Port 993 uses SSL/TLS to secure communication by default, making it the preferred choice for safe, server-retained email access.

Here’s an evergreen article from Team Nuggets called 5 Threats to Your Email Server. As the article says, everything feeds into email, and they’re an easy target for hackers.

Security Concerns of Port 143

Security is the primary issue with port 143. In the same way that the unsecure telnet protocol (port 23) was superseded by SSH (port 22), IMAP is now considered deprecated in favor of IMAP over SSL/TLS. Here are some of the security issues related to port 143:

Unencrypted Data Transmission

The IMAP data sent on port 143 is unencrypted—it’s plaintext, which makes it vulnerable to eavesdropping.

Man-in-the-Middle (MITM) Attacks

Also known as on-path attacks, MITM occurs when someone intercepts a data stream without permission. This can happen in public places like cafes. An MITM attacker can potentially capture and modify data or steal sensitive information.

Password Sniffing

Since data is passing through unencrypted, attackers could read plaintext passwords as they are entered. Capturing login credentials can allow unauthorized access to email accounts.

Other Exploits

An exposed port 143 is just an opening for a skilled attacker. Once they get in, they could use many other potential hacks to exploit systems and networks.

Port 143 FAQs

What’s the Difference Between IMAP and POP3?

IMAP stores email on the server, while POP3 downloads emails from the server to the client device, often deleting them from the server. IMAP supports folder organization and synchronization, but POP3 does not. See the table below for the port assignments.

Can I Safely Use Port 143?

IMAP on port 143 still works, but it won’t have the robust security of IMAPS on port 993. You can upgrade to an encrypted connection using STARTTLS to protect data transmission. However, while IMAP with STARTTLS can be secure if implemented correctly and used with best practices, IMAPS remains the better option.

How Does Port 143 Impact Email Syncing?

IMAP enables real-time email syncing across multiple devices. It also supports folder organization and synchronization, allowing users to manage emails efficiently. IMAP also updates email status (e.g., read/unread, deleted) across all connected devices.

Conclusion

Port 143, used for IMAP, enables email clients to access and manage emails on remote servers. A few decades ago, users and techs were not really aware of the significant security risks posed by unsecured protocols like IMAP, telnet, and file transfer protocol (FTP). 

However, the proper implementation of STARTTLS with IMAP, or better yet, switching to IMAPS, can take advantage of the email capabilities of the port 143 protocol without sacrificing security.

Want to learn more about how email fits in the TCP/IP stack? Be sure to check out this video from CBT Nuggets instructor Keith Barker.