Merge branch 'crypto' into 'master'

postfix: allow client to select the preferred cipher

See merge request simple-nixos-mailserver/nixos-mailserver!412

mail-server/postfix.nix

@@ -287,10 +287,12 @@ in
         smtp_tls_mandatory_exclude_ciphers = "MD5, DES, ADH, RC4, PSD, SRP, 3DES, eNULL, aNULL";
         smtp_tls_exclude_ciphers = "MD5, DES, ADH, RC4, PSD, SRP, 3DES, eNULL, aNULL";
 
-        tls_preempt_cipherlist = true;
+        # As long as all cipher suites are considered safe, let the client use its preferred cipher
+        tls_preempt_cipherlist = false;
 
         # Allowing AUTH on a non encrypted connection poses a security risk
         smtpd_tls_auth_only = true;
+
         # Log only a summary message on TLS handshake completion
         smtp_tls_loglevel = "1";
         smtpd_tls_loglevel = "1";