Port 143 is the standard default port number assigned to the Internet Message Access Protocol (IMAP).
IMAP is an application layer internet protocol used by email clients to retrieve email messages from a mail server over a TCP/IP connection. Unlike older protocols like POP3 (Post Office Protocol version 3), IMAP allows users to manage their emails directly on the server. This means actions like organizing messages into folders, marking emails as read or unread, or deleting emails are synchronized across all devices accessing the same account.
Port 143 in Detail
According to standard port assignments, specifically the information from the Wikipedia list of TCP and UDP port numbers:
| Port | TCP | Description |
|---|---|---|
| 139 | Yes | NetBIOS Session Service |
| 143 | Yes | Internet Message Access Protocol (IMAP), management of electronic mail messages on a server |
| 151 | Assigned | HEMS |
| 152 | Yes | Background File Transfer Program (BFTP) |
This table confirms that Port 143 is a TCP port and is designated for IMAP, facilitating the "management of electronic mail messages on a server."
How IMAP Uses Port 143
When you set up an email client (like Outlook, Thunderbird, Apple Mail, or a mobile email app) using the IMAP protocol, the client typically attempts to connect to your mail server's IMAP service on port 143.
Here's a simple breakdown:
- Client Initiation: Your email client initiates a connection to the mail server.
- Port Selection: It specifies port 143 for the connection, indicating its intention to use the standard IMAP protocol.
- Secure Alternative: It's important to note that while port 143 is the standard non-encrypted IMAP port, secure IMAP communication (IMAPS) typically uses port 993 with SSL/TLS encryption. Port 143 can sometimes be used with STARTTLS, which upgrades an insecure connection to a secure one.
- Communication: Once the connection is established (either plain on 143 or upgraded with STARTTLS), the client sends commands to the server to list mailboxes, fetch messages, mark emails, etc., and the server responds.
Using port 143 allows email clients to access and manage emails stored centrally on the mail server, providing a consistent view of the mailbox across different devices.
