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Document email authentication requirements

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link2xt 2024-04-10 17:46:18 +00:00 committed by missytake
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README.md
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@ -159,4 +159,27 @@ While this file is present, account creation will be blocked.
Delta Chat apps will, however, discover all ports and configurations
automatically by reading the [autoconfig XML file](https://www.ietf.org/archive/id/draft-bucksch-autoconfig-00.html) from the chatmail service.
## Email authentication
chatmail servers rely on [DKIM](https://www.rfc-editor.org/rfc/rfc6376)
to authenticate incoming emails.
Incoming emails must have a valid DKIM signature with
Signing Domain Identifier (SDID, `d=` parameter in the DKIM-Signature header)
equal to the `From:` header domain.
This property is checked by OpenDKIM screen policy script
before validating the signatures.
This correpsonds to strict [DMARC](https://www.rfc-editor.org/rfc/rfc7489) alignment (`adkim=s`),
but chatmail does not rely on DMARC and does not consult the sender policy published in DMARC records.
Other legacy authentication mechanisms such as [iprev](https://www.rfc-editor.org/rfc/rfc8601#section-2.7.3)
and [SPF](https://www.rfc-editor.org/rfc/rfc7208) are also not taken into account.
If there is no valid DKIM signature on the incoming email,
the sender receives a "5.7.1 No valid DKIM signature found" error.
Outgoing emails must be sent over authenticated connection
with envelope MAIL FROM (return path) corresponding to the login.
This is ensured by Postfix which maps login username
to MAIL FROM with
[`smtpd_sender_login_maps`](https://www.postfix.org/postconf.5.html#smtpd_sender_login_maps)
and rejects incorrectly authenticated emails with [`reject_sender_login_mismatch`](reject_sender_login_mismatch) policy.
`From:` header must correspond to envelope MAIL FROM,
this is ensured by `filtermail` proxy.