Can I add "smtpd_milters = inet:127.0.0.1:8891,........" to postfix main.cf from ISPconfig 3.2.2 to enable opendkim?
No, as it makes no sense to do that. ISPConfig uses Amavis or Rspamd for Dkim signing and not OpenDkim. Just enable Dkim in the email domain settings in ISPConfig.
By default, Dkim signing is done by amavis, so your system is already setup to sign emails with dkim if you followed one of the perfect server install guides, installing Rspamd is not required for that and there needs nothing to be enabled. Just switch it on in email domain settings in ISPConfig as I mentioned already, it works out of the boix without requiring any additional software to be installed or enabled beside what's installed in any ISPConfig perfect server install guide. If you want to replace amavis with rspamd (not relevant for Dkim signing), then have a look here: https://www.howtoforge.com/replacing-amavisd-with-rspamd-in-ispconfig/
Already using Rspamd based on that guide. So I can just enable dkim under email -> domain and add the dreated DNS record in my separate DNS server? Then Rspamd will do the signing?
