nixbld: enable mail server again

nixbld-etc-nixos/configuration.nix

@@ -19,6 +19,10 @@ in
       ./github-backup-module.nix
       ./afws-module.nix
       ./rt.nix
+      (builtins.fetchTarball {
+        url = "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/archive/15cf252a0dfffb2420a60fbd6be3467c11026142/nixos-mailserver-nixos.tar.gz";
+        sha256 = "sha256:07c844h8iw0l6l1d5cb29qmfa7nwzxbwq5llnyk6hh913h775csc";
+      })
     ];
 
   boot.loader.grub.enable = true;
@@ -825,5 +829,17 @@ in
     };
   };
 
+  mailserver = {
+    enable = true;
+    localDnsResolver = false;  # conflicts with dnsmasq
+    # Some mail servers do reverse DNS lookups to filter spam.
+    # Getting a proper reverse DNS record from ISP is difficult, so use whatever already exists.
+    fqdn = "42-200-147-171.static.imsbiz.com";
+    domains = [ "nmigen.org" ];
+    loginAccounts = (import /etc/nixos/secret/email_accounts.nix);
+    certificateScheme = 3;
+  };
+  security.acme.certs."${config.mailserver.fqdn}".extraDomainNames = [ "mail.nmigen.org" ];
+
   system.stateVersion = "21.05";
 }