- Require TLS and authenticated submission.
- Make sure the server is not an open relay.
- Publish SPF, DKIM and DMARC for the domain.
Port 406: Interactive Mail Support Protocol
Port 406 is assigned by IANA to imsp (Interactive Mail Support Protocol).
406
How imsp works
EmailMail servers hand messages to each other over port 406, 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 406 dangerous?
We have no specific security note for port 406. Its risk depends on the program actually listening on it. General advice for email services:
IANA registry entries for port 406
| Service | Port | Transport | Description | Reference | Updated |
|---|---|---|---|---|---|
| imsp | 406 | TCP, UDP | Interactive Mail Support Protocol | - | - |
// source: IANA service name and port number registry
Which program uses port 406, and is it reachable?
Linux
sudo ss -ltnup 'sport = :406' # process listening on 406
sudo lsof -nP -i :406macOS
sudo lsof -nP -i :406
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 406 -State Listen | Select LocalAddress, OwningProcess Get-NetUDPEndpoint -LocalPort 406 netstat -ano | findstr :406
From another machine
nmap -p 406 -sV <host> sudo nmap -sU -p 406 <host> nc -zv <host> 406
Block it in the firewall
sudo ufw deny 406/tcp # Linux (ufw) New-NetFirewallRule -DisplayName "Block 406" -Direction Inbound -LocalPort 406 -Protocol TCP -Action Block # Windows
What is port 406 used for?
Port 406 is assigned by IANA to imsp (Interactive Mail Support Protocol).
Is port 406 TCP or UDP?
Port 406 is registered for TCP and UDP.
Is it safe to open port 406?
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 406 is open?
On Linux run "ss -ltnup 'sport = :406'", on macOS "lsof -nP -i :406", and on Windows "netstat -ano | findstr :406" to see the local process. From another machine, "nmap -p 406 <host>" shows whether it is reachable.