Port 3905 MUPDATE
Port 3905 is the IANA-assigned mupdate port for Mailbox Update (MUPDATE) protocol (TCP / UDP).
What is port 3905 used for?
Port 3905 is assigned to mupdate and uses TCP / UDP. The IANA registry describes the service as “Mailbox Update (MUPDATE) protocol”. It sits in the registered range, the block used by databases, application servers and developer tooling. Port conflicts here are common when several local services start at once.
Ports are the transport-layer address a service listens on. A client that wants to reach mupdate connects to port 3905. A TCP connection is refused when nothing is listening, while a UDP request simply goes unanswered. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 3905 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.
Source: IANA Service Name and Transport Protocol Port Number Registry. Reference: [RFC 3656].
Check and block port 3905
Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.
Check on Linux
sudo ss -tlpn 'sport = :3905'
sudo ss -ulpn 'sport = :3905'Check on Windows
Get-NetTCPConnection -LocalPort 3905 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 3905 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 3905/tcp
sudo ufw deny 3905/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 3905 -j DROP
sudo iptables -A INPUT -p udp --dport 3905 -j DROPThese commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.
Is port 3905 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
Where port 3905 is usually used
Most traffic on port 3905 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Mail clients sending messages through a relay or fetching them from a mailbox.
- Mail servers exchanging messages for a domain or syncing folders with clients.
- Deliverability, authentication and TLS tests that connect directly to the mail service.
Test the mail listener
Connect directly so the client configuration cannot hide a wrong hostname or blocked port.
nc -vz mail.example.com 3905Common problems with port 3905
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 3905. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.
Port already in use
Find the process holding the port with sudo ss -tlpn 'sport = :3905' or sudo lsof -i :3905. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.
Firewall rule did not survive a reboot
UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.
The port is blocked but still reachable
A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.
Port 3905 at a glance
| Port number | 3905 |
|---|---|
| Service name | mupdate |
| Transport protocol | TCP / UDP |
| Category | |
| Range | Registered port (1024–10000) |
| Risk level | Standard/Low |
| Registered assignee | Not listed |
| Reference | [RFC 3656] |
| Registry description | Mailbox Update (MUPDATE) protocol |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 3905 used for?
Port 3905 (MUPDATE) uses TCP / UDP. Mailbox Update (MUPDATE) protocol.
How do I check if port 3905 is open?
On Linux run `sudo ss -tlpn 'sport = :3905'` and `sudo ss -ulpn 'sport = :3905'`. On Windows run `Get-NetTCPConnection -LocalPort 3905 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 3905 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 3905; empty output usually means nothing is bound to it.
How do I block port 3905?
With UFW run `sudo ufw deny 3905/tcp` and `sudo ufw deny 3905/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 3905 -j DROP` and `sudo iptables -A INPUT -p udp --dport 3905 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 3905 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Nevertheless, keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
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Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy