postfix: configure preferred curves and disable FFDHE

This aligns with the intermediate configuration recommended by Mozilla.

mail-server/postfix.nix

@@ -296,6 +296,20 @@ in
         smtp_tls_exclude_ciphers = "eNULL, aNULL";
         smtp_tls_mandatory_exclude_ciphers = "eNULL, aNULL";
 
+        # Restrict and prioritize the following curves in the given order
+        # Excludes curves that have no widespread support, so we don't bloat the handshake needlessly.
+        # https://www.postfix.org/postconf.5.html#tls_eecdh_auto_curves
+        # https://ssl-config.mozilla.org/#server=postfix&version=3.10&config=intermediate&openssl=3.4.1&guideline=5.7
+        tls_eecdh_auto_curves = [
+          "X25519"
+          "prime256v1"
+          "secp384r1"
+        ];
+
+        # Disable FFDHE on TLSv1.3 because it is slower than elliptic curves
+        # https://www.postfix.org/postconf.5.html#tls_ffdhe_auto_groups
+        tls_ffdhe_auto_groups = [ ];
+
         # As long as all cipher suites are considered safe, let the client use its preferred cipher
         tls_preempt_cipherlist = false;