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Port 50 (re-mail-ck)

IANA assigns port 50 to re-mail-ck: Remote Mail Checking Protocol.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
re-mail-ck TCP Remote Mail Checking Protocol Steve Dorner -
re-mail-ck UDP Remote Mail Checking Protocol Steve Dorner -

Check what is listening on port 50

Linux: is anything listening on TCP 50?
sudo ss -tlnp 'sport = :50'
Linux: UDP 50
sudo ss -ulnp 'sport = :50'
Linux / macOS: which process owns the port
sudo lsof -nP -i :50
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':50 '
Windows (Command Prompt)
netstat -ano | findstr :50
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 50 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 50
From Windows
Test-NetConnection HOST -Port 50
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 50

Block port 50 in the firewall

ufw
sudo ufw deny 50/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 50 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 50 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 50' -Direction Inbound -Protocol TCP -LocalPort 50 -Action Block
ufw
sudo ufw deny 50/udp
iptables
sudo iptables -A INPUT -p udp --dport 50 -j DROP
nftables
sudo nft add rule inet filter input udp dport 50 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 50' -Direction Inbound -Protocol UDP -LocalPort 50 -Action Block

Related ports

48auditd49TACACS+52xns-time53DNS

All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.