mail-server: add dmarcReporting.excludeDomains

The option `exclude_domains` for dmarc reporting in `rspamd`[1] allows
to configure a list of domains and/or eSLDs (external effective second level
domain) to be excluded from dmarc reports.

Helpful because e.g. dmarc reports to hotmail.com always fail for me
with the following undeliverable notification:

    The recipient's mailbox is full and can't accept messages now.

[1] https://www.rspamd.com/doc/modules/dmarc.html

mail-server/rspamd.nix

@@ -74,6 +74,9 @@ in
                 org_name = "${cfg.dmarcReporting.organizationName}";
                 from_name = "${cfg.dmarcReporting.fromName}";
                 msgid_from = "dmarc-rua";
+                ${lib.optionalString (cfg.dmarcReporting.excludeDomains != []) ''
+                  exclude_domains = ${builtins.toJSON cfg.dmarcReporting.excludeDomains};
+                ''}
               }''}
           ''; };
       };