mailcow-dockerized/data/conf/rspamd/local.d/dkim.conf

sign_condition =<<EOD
return function(task)
  local from = task:get_from('smtp')
  local rspamd_logger = require "rspamd_logger"
  if not task:get_user() then
    rspamd_logger.infox(task, "Skip DKIM signing for unauthorized user")
    return false
  end
  if from and from[1]['addr'] then
    lastAtSymbol = from[1]['addr']:find("[^%@]+$")
    local domain = from[1]['addr']:sub(lastAtSymbol, #from[1]['addr'])
    local keyfile = io.open("/data/dkim/keys/" .. domain .. ".dkim")
    if keyfile then
      rspamd_logger.infox(task, "DKIM signing mail for authorized user of domain %s", domain)
      keyfile:close()
      return {
        key = "/data/dkim/keys/" .. domain .. ".dkim",
        domain = domain,
        selector = "dkim"
      }
    else
      rspamd_logger.infox(task, "Authorized user, but no key file for domain %s - skipped DKIM signing", domain)
    end
  end
  return false
end
EOD;