Your mail app keeps a long-lived connection to port 993 so new messages appear instantly (IMAP IDLE). In a network monitor, that shows up as a steady connection from Mail to your provider with little traffic until a message arrives. Several connections per account are normal.
Facts
- Port
- 993
- Protocols
- TCP
- Service
- IMAPS
What uses port 993
Email clients reading mail from the server with IMAP, encrypted with TLS from the first byte. Unencrypted IMAP uses 143.
On a Mac
Apple Mail and other clients connect to 993 for IMAP accounts such as iCloud Mail, Gmail and Fastmail. If mail stops arriving on one network, check that 993 isn't blocked.
sudo lsof -nP -iTCP:993Security
993 is encrypted; prefer it over 143. Use app-specific passwords or OAuth where the provider supports them.
Frequently asked questions
What is the difference between port 993 and 143?
993 is IMAP with TLS from the start. 143 is plain IMAP, which may upgrade with STARTTLS. Use 993 when available.
Does iCloud Mail use port 993?
Yes. iCloud Mail's incoming server is imap.mail.me.com on port 993 with SSL.
Why does Mail keep a connection open?
IMAP IDLE keeps a connection to the server so new messages are pushed immediately instead of checked periodically.