Port 406Interactive Mail Support Protocol

Port 406 is assigned to the Interactive Mail Support Protocol for interactive mail services.

transport: tcp · udp
in transit: cleartext
assignment: official
risk: 2/10
lookups: 0

also known as imsp

Technical Details

what runs on :406

IMSP is an application-layer request/response protocol for accessing and modifying mail-support data rather than transferring messages. IANA assigns it to both TCP and UDP port 406, although practical deployments are generally associated with a session-oriented TCP service. It predates newer directory and mail-configuration protocols such as ACAP and is normally encountered only on legacy mail infrastructure.

Security Information

exposure of :406

risk score

2/ 10safe

routine exposure. this port is rarely the way in on its own — keep it patched and logged and move on.

email averages 3.2 across 62 ports — this one sits 1.2 below.

in transit

cleartext

credentials and payloads are readable by anyone on path

assignment

official

registered with iana for this service — scanners fingerprint it by number

reachable over

tcp · udp

udp is connectionless — source addresses are trivially spoofed and it is a reflection favourite

security overview

Legacy IMSP traffic should not be exposed directly to the internet. The protocol does not provide modern transport encryption by default, so credentials and mail-profile data may be exposed to interception or manipulation unless protected by an external secure tunnel; restrict port 406 to trusted clients and replace obsolete deployments where possible.

Record

iana registry entry

port: :406
service: Interactive Mail Support Protocol
transport: TCPUDP
encryption: cleartext
assignment: iana registered
risk: 2/10safe
category: Email
usage: IMSP is a legacy client/server protocol for managing mail-related user information such as profiles, aliases, and preferences. It is rarely encountered today; modern deployments generally use other mail, directory, or configuration protocols instead.
lookups: 0rank 993
added: 2026-08-26
updated: 2026-09-22
references: IANA service name registry — port 406

Something wrong?

This record is maintained by the engineers who use port 406. Every field on it is editable — change what's wrong, say how you know, and a reviewer publishes it, usually within a minute. Last touched 2026-09-22.

01edit any field on a page of its own
02a model checks it for accuracy and spam
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edit this record →back to the registry

Related Ports

all 62 in email →

the 8 most looked-up other ports in email — 62 ports carry that label.

port	service	proto	category	risk	views
:143	IMAP	TCPUDP	Email	caution	20.7k
:995	POP3S	TCPtls	Email	caution	18.8k
:109	POP2	TCP	Email	caution	17.9k
:2096	cPanel SSL Webmail	TCPtls	Web Services	safe	17.4k
:110	POP3	TCP	Email	caution	15.5k
:993	IMAPS	TCPtls	Email	caution	14.5k
:1352	Lotus Notes RPC	TCP	Email	caution	12.3k
:24	Private Mail	TCPUDP	Email	caution	11.7k

risk mix of the 8 listed

safe13%
caution88%

3 of 8 encrypted

Port :406Interactive Mail Support Protocol

Technical Details

Security Information

Related Ports

Port 406Interactive Mail Support Protocol