# WHATPORT(406)

## NAME

port 406 — IMSP (Interactive Mail Support Protocol) — A dead companion protocol to IMAP that stored address books and mailbox subscriptions on the server.

Protocol: TCP  
IANA: imsp

## DESCRIPTION

IMSP came out of Carnegie Mellon's Cyrus mail project to hold the things IMAP did not: personal address books, per-user options, and mailbox subscription lists shared across clients. It never became an IETF standard; the working group's successor effort, ACAP (RFC 2244), was published but saw almost no adoption either, and both were eventually made moot by IMAP extensions and by LDAP for directories. IANA still holds 406 for imsp, and a listener there means a surviving Cyrus-era deployment.

## EXPOSURE

**Do not expose.** IMSP authenticates in cleartext by default and its implementations are unmaintained — there is no version of this that belongs on a public address.

## CHECK

```sh
nc -vz -w 3 TARGET 406
```

## SEE ALSO

- spec — [RFC 2244 — ACAP, the standardised successor to IMSP](https://www.rfc-editor.org/rfc/rfc2244.html) (rfc-editor.org)
- iana — [IANA Service Name and Transport Protocol Port Number Registry — 406](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=406) (iana.org)
- source — [cyrusimap/cyrus-imapd — the Cyrus mail project IMSP came from](https://github.com/cyrusimap/cyrus-imapd) (github.com)

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HTML: <https://whatport.net/406/> · JSON: <https://whatport.net/406.json> · full dataset: <https://whatport.net/ports.json>