diff --git a/nixos/tests/postfix.nix b/nixos/tests/postfix.nix
index 29e263cd7e7..6d22b4edba0 100644
--- a/nixos/tests/postfix.nix
+++ b/nixos/tests/postfix.nix
@@ -11,9 +11,9 @@ import ./make-test-python.nix {
       enable = true;
       enableSubmission = true;
       enableSubmissions = true;
-      tlsTrustedAuthorities = certs.ca.cert;
-      sslCert = certs.${domain}.cert;
-      sslKey = certs.${domain}.key;
+      tlsTrustedAuthorities = "${certs.ca.cert}";
+      sslCert = "${certs.${domain}.cert}";
+      sslKey = "${certs.${domain}.key}";
       submissionsOptions = {
           smtpd_sasl_auth_enable = "yes";
           smtpd_client_restrictions = "permit";
diff --git a/pkgs/servers/mail/postfix/default.nix b/pkgs/servers/mail/postfix/default.nix
index 9aee89db9b7..a4346bdbd2b 100644
--- a/pkgs/servers/mail/postfix/default.nix
+++ b/pkgs/servers/mail/postfix/default.nix
@@ -1,6 +1,6 @@
 { stdenv, lib, fetchurl, makeWrapper, gnused, db, openssl, cyrus_sasl, libnsl
 , coreutils, findutils, gnugrep, gawk, icu, pcre, m4
-, buildPackages
+, buildPackages, nixosTests
 , withLDAP ? true, openldap
 , withPgSQL ? false, postgresql
 , withMySQL ? false, libmysqlclient
@@ -96,6 +96,8 @@ in stdenv.mkDerivation rec {
       --prefix PATH ":" ${lib.makeBinPath [ coreutils findutils gnugrep gawk gnused ]}
   '';
 
+  passthru.tests = { inherit (nixosTests) postfix postfix-raise-smtpd-tls-security-level; };
+
   meta = with lib; {
     homepage = "http://www.postfix.org/";
     description = "A fast, easy to administer, and secure mail server";