diff --git a/default.nix b/default.nix
index 60dbceb..88e1829 100644
--- a/default.nix
+++ b/default.nix
@@ -41,7 +41,15 @@ in
       type = types.listOf types.str;
       example = [ "example.com" ];
       default = [];
-      description = "The domains that this mail server serves.";
+      description = "The domains that this mail server serves and provides a mailbox for.";
+    };
+
+    relayDomains = mkOption {
+      type = types.listOf types.str;
+      example = [ "lists.example.com" ];
+      default = [];
+      defaultText = lib.literalExpression "config.mailserver.domains";
+      description = "The domains that this mail server relays, in addition to those in {option}`domains`.";
     };
 
     certificateDomains = mkOption {
@@ -1319,4 +1327,8 @@ in
     ./mail-server/nginx.nix
     ./mail-server/kresd.nix
   ];
+
+  config = {
+     mailserver.relayDomains = cfg.domains;
+  };
 }
diff --git a/flake.nix b/flake.nix
index 6fb5637..2b9a497 100644
--- a/flake.nix
+++ b/flake.nix
@@ -34,12 +34,13 @@
       "clamav"
       "multiple"
       "ldap"
+      "relay"
     ];
     genTest = testName: release: {
       "name"= "${testName}-${builtins.replaceStrings ["."] ["_"] release.name}";
       "value"= import (./tests/. + "/${testName}.nix") {
         pkgs = release.pkgs;
-        inherit blobs;
+        inherit blobs lib;
       };
     };
     # Generate an attribute set such as
diff --git a/mail-server/opendkim.nix b/mail-server/opendkim.nix
index cdb283c..5fd096d 100644
--- a/mail-server/opendkim.nix
+++ b/mail-server/opendkim.nix
@@ -41,13 +41,13 @@ let
               echo "Generated key for domain ${dom} selector ${cfg.dkimSelector}"
           fi
         '';
-  createAllCerts = lib.concatStringsSep "\n" (map createDomainDkimCert cfg.domains);
+  createAllCerts = lib.concatStringsSep "\n" (map createDomainDkimCert cfg.relayDomains);
 
   keyTable = pkgs.writeText "opendkim-KeyTable"
-    (lib.concatStringsSep "\n" (lib.flip map cfg.domains
+    (lib.concatStringsSep "\n" (lib.flip map cfg.relayDomains
       (dom: "${dom} ${dom}:${cfg.dkimSelector}:${cfg.dkimKeyDirectory}/${dom}.${cfg.dkimSelector}.key")));
   signingTable = pkgs.writeText "opendkim-SigningTable"
-    (lib.concatStringsSep "\n" (lib.flip map cfg.domains (dom: "${dom} ${dom}")));
+    (lib.concatStringsSep "\n" (lib.flip map cfg.relayDomains (dom: "${dom} ${dom}")));
 
   dkim = config.services.opendkim;
   args = [ "-f" "-l" ] ++ lib.optionals (dkim.configFile != null) [ "-x" dkim.configFile ];
@@ -58,7 +58,7 @@ in
         enable = true;
         selector = cfg.dkimSelector;
         keyPath = cfg.dkimKeyDirectory;
-        domains = "csl:${builtins.concatStringsSep "," cfg.domains}";
+        domains = "csl:${builtins.concatStringsSep "," cfg.relayDomains}";
         configFile = pkgs.writeText "opendkim.conf" (''
           Canonicalization ${cfg.dkimHeaderCanonicalization}/${cfg.dkimBodyCanonicalization}
           UMask 0002
diff --git a/mail-server/postfix.nix b/mail-server/postfix.nix
index 6ba6ec6..964ba71 100644
--- a/mail-server/postfix.nix
+++ b/mail-server/postfix.nix
@@ -157,6 +157,7 @@ let
     tls_ca_cert_file = ${cfg.ldap.tlsCAFile}
     tls_require_cert = yes
 
+    domain = ${lib.concatStringsSep ", " cfg.domains}
     search_base = ${cfg.ldap.searchBase}
     scope = ${cfg.ldap.searchScope}
 
diff --git a/mail-server/rspamd.nix b/mail-server/rspamd.nix
index 8fb9b00..42e4d43 100644
--- a/mail-server/rspamd.nix
+++ b/mail-server/rspamd.nix
@@ -73,7 +73,7 @@ in
                 domain = "${cfg.dmarcReporting.domain}";
                 org_name = "${cfg.dmarcReporting.organizationName}";
                 from_name = "${cfg.dmarcReporting.fromName}";
-                msgid_from = "dmarc-rua";
+                msgid_from = "${cfg.dmarcReporting.domain}";
               }''}
           ''; };
       };
diff --git a/tests/clamav.nix b/tests/clamav.nix
index ae186df..3828a72 100644
--- a/tests/clamav.nix
+++ b/tests/clamav.nix
@@ -14,7 +14,7 @@
 #  You should have received a copy of the GNU General Public License
 #  along with this program. If not, see <http://www.gnu.org/licenses/>
 
-{ pkgs ? import <nixpkgs> {}, blobs}:
+{ pkgs ? import <nixpkgs> {}, blobs, ...}:
 
 pkgs.nixosTest {
   name = "clamav";
diff --git a/tests/relay.nix b/tests/relay.nix
new file mode 100644
index 0000000..3088cc3
--- /dev/null
+++ b/tests/relay.nix
@@ -0,0 +1,78 @@
+# This tests is used to test features requiring several mail domains.
+
+{ lib, pkgs ? import <nixpkgs> { }, ... }:
+
+let
+  hashPassword = password: pkgs.runCommand
+    "password-${password}-hashed"
+    { buildInputs = [ pkgs.mkpasswd ]; inherit password; }
+    ''
+      mkpasswd -sm bcrypt <<<"$password" > $out
+    '';
+
+  password = pkgs.writeText "password" "password";
+
+  domainGenerator = domain: {
+    imports = [ ../default.nix ];
+    virtualisation.memorySize = 1024;
+    mailserver = {
+      enable = true;
+      fqdn = "mail.${domain}";
+      domains = [ domain ];
+      localDnsResolver = false;
+      loginAccounts = {
+        "user@${domain}" = {
+          hashedPasswordFile = hashPassword "password";
+        };
+      };
+      enableImap = true;
+      enableImapSsl = true;
+    };
+    services = {
+      dnsmasq = {
+        enable = true;
+        settings.mx-host = [ "domain1.com,domain1,10" "domain2.com,domain2,10" ];
+      };
+      # disable rspamd because of graylisting
+      postfix.config.smtpd_milters = lib.mkForce [ ];
+      rspamd.enable = lib.mkForce false;
+      redis.servers.rspamd.enable = false;
+    };
+    systemd.services.postfix.requires = lib.mkForce [ "postfix-setup.service" ];
+  };
+
+in
+
+pkgs.nixosTest {
+  name = "relay";
+  nodes = {
+    domain1 = {
+      imports = [
+        ../default.nix
+        (domainGenerator "domain1.com")
+      ];
+      mailserver.relayDomains = [ "replay.domain1.com" ];
+      # ip of itself
+      services.postfix.networks = [ "[2001:db8:1::1]/128" ];
+    };
+    domain2 = domainGenerator "domain2.com";
+    client = { pkgs, ... }: {
+      environment.systemPackages = [
+        (pkgs.writeScriptBin "mail-check" ''
+          ${pkgs.python3}/bin/python ${../scripts/mail-check.py} $@
+        '')
+      ];
+    };
+  };
+  testScript = ''
+    start_all()
+
+    domain1.wait_for_unit("multi-user.target")
+    domain2.wait_for_unit("multi-user.target")
+
+    # user@domain1.com sends a mail to user@domain2.com
+    client.succeed(
+        "mail-check send-and-read --smtp-port 25 --smtp-starttls --smtp-host domain1 --from-addr user@relay.domain1.com --imap-host domain2 --to-addr user@domain2.com --dst-password-file ${password} --ignore-dkim-spf"
+    )
+  '';
+}