Port 143: Which Email Protocol Uses It?
Port number 143 is the well-known TCP port used for IMAP (Internet Message Access Protocol) over plaintext (i.e., non-TLS) connections. In typical email client/server configurations, IMAP uses TCP 143, while IMAPS uses TCP 993 (IMAP over TLS/SSL).2
To answer your multiple-choice question: (ii) IMAP.
Key terms:
- port number
- IMAP
- TCP
- TLS
Footnotes
-
IANA Port 143 - imap - IANA service/port registry listing IMAP on port 143. ↩
-
RFC 3501 IMAP version 4rev1 - Defines IMAP and references its standard port usage. ↩
IMAP vs POP3 (Email Protocols Overview)
Email-related “standard ports” often appear in firewall rules and client configuration wizards. The most common ones are:
- SMTP
- POP3
- FTP
- IMAP
Common default ports (high level)
| Protocol | Typical use | Common port(s) |
|---|---|---|
| SMTP | Sending mail | 25, 587 (submission) |
| IMAP | Reading/managing mail on server | 143 |
| POP3 | Downloading mail from server | 110 |
| FTP | File transfer | 21 (control), 20 (data) |
These align with IANA’s service-name/port registry for the “well-known” ports used by many systems.2
Footnotes
-
IANA Port 143 - imap - IANA service/port registry listing IMAP on port 143. ↩
-
IANA Service Names and Transport Protocols Registry - Official IANA registry for common protocol-to-port mappings. ↩
How to interpret a port number like 143
- 1Step 1
IMAP commonly refers to TCP port 143; confirm whether your rule/firewall is TCP or UDP.
- 2Step 2
Check IANA’s service-name/port registry to see which service is assigned to that port.
- 3Step 3
IMAP on 143 is typically plaintext; encrypted IMAP commonly uses a different port (e.g., 993 for IMAPS).
- 4Step 4
Email clients typically let you choose IMAP with port 143 (or IMAPS with port 993), matching the server’s configuration.
Pro Tip
When configuring an email client, always match both the protocol (IMAP) and the port (143 for IMAP plaintext). If you enable “SSL/TLS,” the port may need to change (commonly to 993).
Common mistake
Port 143 does not refer to SMTP, POP, or FTP. If you see 143 in an email context, it’s almost always IMAP (not POP3). Misconfigured ports cause authentication/timeouts or “cannot connect” errors.
Quick FAQ
Typical email access flow (IMAP)
Client connects to server
1) ConnectClient opens a TCP connection to port 143 (IMAP plaintext)."
Login/authentication
2) AuthenticateClient authenticates (mechanism depends on server configuration)."
Choose folder
3) Select mailboxClient selects a mailbox (e.g., INBOX) to operate on."
Read/manage messages
4) Access messagesClient fetches headers/body and can flag messages without downloading everything."
Knowledge Check
What does port number 143 refer to?