- Require TLS and authenticated submission.
- Make sure the server is not an open relay.
- Publish SPF, DKIM and DMARC for the domain.
Port 50: Remote Mail Checking Protocol
Port 50 is assigned by IANA to re-mail-ck (Remote Mail Checking Protocol).
50
How re-mail-ck works
EmailMail servers hand messages to each other over port 50, one hop at a time, until the recipient's server.
- Transport
- TCP · UDP
- Exposure risk
- not rated depends on the program listening
- Encryption
- Unknownnot stated by the registry
- IANA status
- Assigned1 registered service
- Range
- Well-known (0-1023)
- Category
Is port 50 dangerous?
We have no specific security note for port 50. Its risk depends on the program actually listening on it. General advice for email services:
IANA registry entries for port 50
| Service | Port | Transport | Description | Reference | Updated |
|---|---|---|---|---|---|
| re-mail-ck | 50 | TCP, UDP | Remote Mail Checking Protocol | - | - |
// source: IANA service name and port number registry
Which program uses port 50, and is it reachable?
Linux
sudo ss -ltnup 'sport = :50' # process listening on 50
sudo lsof -nP -i :50macOS
sudo lsof -nP -i :50
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 50 -State Listen | Select LocalAddress, OwningProcess Get-NetUDPEndpoint -LocalPort 50 netstat -ano | findstr :50
From another machine
nmap -p 50 -sV <host> sudo nmap -sU -p 50 <host> nc -zv <host> 50
Block it in the firewall
sudo ufw deny 50/tcp # Linux (ufw) New-NetFirewallRule -DisplayName "Block 50" -Direction Inbound -LocalPort 50 -Protocol TCP -Action Block # Windows
What is port 50 used for?
Port 50 is assigned by IANA to re-mail-ck (Remote Mail Checking Protocol).
Is port 50 TCP or UDP?
Port 50 is registered for TCP and UDP.
Is it safe to open port 50?
It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Require TLS and authenticated submission.
How do I check if port 50 is open?
On Linux run "ss -ltnup 'sport = :50'", on macOS "lsof -nP -i :50", and on Windows "netstat -ano | findstr :50" to see the local process. From another machine, "nmap -p 50 <host>" shows whether it is reachable.