DKIM and ARC: Setup MTA: Using Rspamd

Requirements

MTA: Postfix, Sendmail, or maybe OpenSMTPD.
Rspamd 3.3 STABLE or later.
You have to choose (or confirm) authserv-id to determine the results of domain validation. See descriptions below for details. In this document mx.example.org is used for example.

Note that, Rspamd also has DKIM signing and ARC sealing capabilities, but we may not configure them: Sympa is responsible for those features.

Thus, basically the configuration of Rspamd we have to do for Sympa is just adding following line to local.d/milter_headers.conf:

use = ["authentication-results"];

For details see the documentation.

See also "MTA integration" in the documentation.

Postfix

Add following settings to main.cf (Note: Replace mx.example.org in below):
```
smtpd_milters = (existing settings) inet:localhost:11332
milter_default_action = accept
milter_macro_daemon_name = mx.example.org
```
If milter_macro_daemon_name is not specified, value of myhostname is used as authserv-id.
Sendmail

Edit sendmail.cf to add following settings:
```
O InputMailFilters=rspamd
Xrspamd, S=inet:11332@localhost
```
Or, if you are generating sendmail.cf from sendmail.mc, add following lines after FEATURE lines:
```
define(`confINPUT_MAIL_FILTERS', `rspamd')
MAIL_FILTER(`rspamd', `S=inet:11332@localhost')
```
Above is equivalent to below:
```
INPUT_MAIL_FILTER(`rspamd', `S=inet:11332@localhost')
```
With sendmail, the authserv-id is the value of j macro in sendmail.cf (in Dj line). If you are generating sendmail.cf from sendmail.mc, it is defined by confDOMAIN_NAME configuration parameter.
OpenSMTPD

It is reported that filter-rspamd (opensmtpd-filter-rspamd package on OpenBSD) may be used to integrate Rspamd with OpenSMTPD. For more details see related documents.

The authserv-id is the value of hostname option in smtpd.conf.

After you finished setting up MTA, test it.