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Port 143 (TCP) is the default port for IMAP, which mail clients use to access mailboxes on a server. The tool shows you the details and generates firewall commands for the port.
Built by Justin Schuessler
Try the commands on your own VPS, with root access and your own firewall.
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Email · Well-known port · OSI layer 7
Read and manage mail in your mailbox on the server. Without TLS, passwords and emails are sent in plain text.
IANA: officially registered as imap
Firewall commands
Open port
sudo ufw allow 143/tcpFor Ubuntu and Debian. ufw creates the rule for both IPv4 and IPv6. If ufw is disabled, allow SSH first (sudo ufw allow 22/tcp), then enable it with sudo ufw enable, or you'll lock yourself out.
On a ZAP-Hosting VPS with an active network firewall, you also need to allow the port in the Customer Panel.
Port 143 explained
Email clients access a mailbox via IMAP on port 143. Emails remain on the server, while folders and read status are synchronized across all devices. That's why IMAP is now the standard when you read your emails on both a computer and a smartphone. Without additional measures, the connection on port 143 is unencrypted, but it can be upgraded to TLS using STARTTLS.
IMAP emerged in the 1980s as an alternative to POP3, which downloads mail and usually deletes it from the server. IMAP4rev1 in RFC 3501 was the standard for a long time; today it is IMAP4rev2 in RFC 9051. IANA lists port 143 as “imap”. RFC 8314 now recommends implicit TLS on port 993 instead of STARTTLS on port 143 for mail clients.
Without encryption, port 143 transmits usernames, passwords and all emails in plain text. Prefer port 993 with implicit TLS. If you must continue offering port 143, require STARTTLS and prevent login without encryption. Also protect logins against password attacks, for example with Fail2ban.
More ports
These ports are often part of the same setup. Click one to see its explanation and firewall commands.
View all well-known ports and the complete IANA list
Port list source: IANA Service Name and Transport Protocol Port Number Registry, as of October 22, 2026. iana.org
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FAQ
Quick answers about port 143.
Both use IMAP. Port 143 starts unencrypted and can be encrypted with STARTTLS; port 993 is encrypted with TLS from the start. For new setups, use port 993.
Only if STARTTLS is required. Without encryption, anyone intercepting the traffic can read your password and mail. Port 993 is safer and simpler.
IMAP keeps mail on the server and syncs it across all devices. POP3 downloads mail to one device and usually deletes it from the server. If you use multiple devices, IMAP is the better choice.
With ufw, sudo ufw allow 143/tcp is enough. With firewalld, run sudo firewall-cmd --permanent --add-port=143/tcp followed by sudo firewall-cmd --reload. First, consider whether port 993 alone would be enough.
Any more questions?
Contact SupportWith a ZAP-Hosting VPS, you have root access, can set up services and firewall rules yourself, and can immediately see which ports are open.
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