DevToolsmith

Port 993: IMAPS

Port 993 (TCP) is used by IMAP over TLS for securely reading email. It is a well-known port (0–1023). On Linux and macOS, binding to it needs root or the CAP_NET_BIND_SERVICE capability.

Service
IMAPS
Protocol
TCP
Range
Well-known (0–1023)
Official
IANA-registered
Exposure
Meant to be public
Category
Email

What uses port 993

IMAPS is IMAP with TLS from the first byte, and the default for Gmail, Outlook.com, iCloud and most mail providers. If a mail app can’t connect, check that it uses port 993 with SSL/TLS (not STARTTLS) and that app passwords or OAuth are set up when two-factor authentication is on.

Should port 993 be open to the internet?

Meant to be public. This port is meant to be reachable by clients on the internet when you run this service publicly.

Check what is listening on port 993

Linux
sudo ss -ltnp 'sport = :993'
macOS
sudo lsof -nP -iTCP:993 -sTCP:LISTEN
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 993 -State Listen
netstat -ano | findstr :993

No output means nothing is listening. If a service should be, check that it is running and which address it binds to: 127.0.0.1 only accepts local connections, 0.0.0.0 or :: accepts them on every interface.

Fix “address already in use” on port 993

Another process already holds the port. Find it with the commands above, then stop that service properly, or end the process:

macOS / Linux
sudo kill $(sudo lsof -t -iTCP:993 -sTCP:LISTEN)
Windows (PowerShell)
Stop-Process -Id (Get-NetTCPConnection -LocalPort 993 -State Listen).OwningProcess

Open port 993 in the firewall

Ubuntu / Debian (ufw)
sudo ufw allow 993/tcp
# Close it again:
sudo ufw delete allow 993/tcp
RHEL / Fedora / Rocky (firewalld)
sudo firewall-cmd --permanent --add-port=993/tcp
sudo firewall-cmd --reload
Windows (PowerShell, as administrator)
New-NetFirewallRule -DisplayName "IMAPS 993/TCP" -Direction Inbound -Protocol TCP -LocalPort 993 -Action Allow

On AWS, Google Cloud and Azure, also allow the port in the security group or network firewall rules; host firewalls and cloud firewalls are separate.

Test port 993 from another machine

macOS / Linux
nc -zv example.com 993
Windows (PowerShell)
Test-NetConnection example.com -Port 993

Related ports

  • Port 143 (IMAP): used by IMAP for reading email that stays on the server.
  • Port 995 (POP3S): used by POP3 over TLS for securely downloading email.
  • Port 465 (SMTPS): used for email submission over implicit TLS (SMTPS).
  • Port 587 (SMTP submission): the standard port for sending email from apps and mail clients.

More email ports

25 (SMTP) , 110 (POP3) . See all ports in the port lookup.

Sources

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Frequently asked questions

What is port 993 used for?

Port 993 is used by IMAP over TLS for securely reading email. IMAPS is IMAP with TLS from the first byte, and the default for Gmail, Outlook.com, iCloud and most mail providers. If a mail app can’t connect, check that it uses port 993 with SSL/TLS (not STARTTLS) and that app passwords or OAuth are set up when two-factor authentication is on.

Is it safe to open port 993?

This port is meant to be reachable by clients on the internet when you run this service publicly. Keep the service behind it up to date and require authentication where it applies.

How do I check if port 993 is open?

On the server, list listening sockets with "sudo ss -ltnp 'sport = :993'" on Linux or "sudo lsof -nP -i :993" on macOS. From another machine, run "nc -zv host 993" or, on Windows, "Test-NetConnection host -Port 993". Connection refused means nothing is listening; a timeout means a firewall is dropping the traffic.