Network ports · 143
143 — IMAP
Port 143 is IMAP, which keeps mail on the server and synchronises folders across devices.
Key facts
| Port | 143 |
|---|---|
| Transport | TCP |
| Service | IMAP |
| Model | Server-side mailbox |
| Encryption | STARTTLS, optional |
| Range | Well-known (0–1023) |
The mailbox lives on the server
IMAP keeps messages and folders on the server, and each client mirrors what it needs. Read flags, folder structure, search and drafts are shared, which is why IMAP is the default for multi-device mail. The trade-off is that the server stores everything and the client depends on connectivity for full folder views.
IDLE lets the server push new-mail notifications over the same connection, which is how clients show mail immediately without polling.
Upgrading to TLS
Port 143 starts in clear text and can upgrade with STARTTLS. If the server advertises it, a client should refuse to authenticate before the upgrade, or the password crosses the network unprotected. IMAPS on port 993 begins with TLS and avoids that window.
Modern clients default to 993. Keep 143 available only if legacy clients require it, and require STARTTLS when you do.
Frequently asked questions
What is the difference between 143 and 993?
143 is IMAP with optional STARTTLS; 993 is IMAP over TLS from the first byte.
Does IMAP download all mail?
It can sync headers and bodies on demand. The mailbox remains authoritative on the server.