Simple-Nixos-Mailserver: Sending emails fails

I've been trying to achieve a working mail setup on nixos by using simple-nixos-mailserver.

  mailserver = {
    enable = true;
    certificateScheme = "acme-nginx";
    enableManageSieve = true;
    fqdn = "email.teatastic.org";
    domains = ["teatastic.org"];
    mailboxes = {
      Drafts = {
        auto = "subscribe";
        specialUse = "Drafts";
      };
      Junk = {
        auto = "subscribe";
        specialUse = "Junk";
      };
      Sent = {
        auto = "subscribe";
        specialUse = "Sent";
      };
      Trash = {
        auto = "no";
        specialUse = "Trash";
      };
    };

    loginAccounts = {
      "[email protected]" = {
        hashedPasswordFile = config.sops.secrets.password.path;
        aliases = ["[email protected]"];
      };
    };

    fullTextSearch = {
      enable = false;
      enforced = "body";
      indexAttachments = true;
      memoryLimit = 512;
    };

    enableImap = true;
    enablePop3 = true;
    enableImapSsl = true;
    enablePop3Ssl = true;

    virusScanning = false;
  };
  services.roundcube = {
    enable = true;
    package = pkgs.roundcube.withPlugins (
      plugins: [
        plugins.carddav
        plugins.contextmenu
        plugins.custom_from
        plugins.persistent_login
        plugins.thunderbird_labels
      ]
    );
    plugins = [
      "attachment_reminder" # Roundcube internal plugin
      "carddav"
      "contextmenu"
      "custom_from"
      "managesieve" # Roundcube internal plugin
      "newmail_notifier" # Roundcube internal plugin
      "persistent_login"
      "thunderbird_labels"
      "zipdownload" # Roundcube internal plugin
    ];
    #dicts = with pkgs.aspellDicts; [en];
    hostName = config.mailserver.fqdn;
    maxAttachmentSize = 100;
    extraConfig = ''
      $config['smtp_server'] = "tls://${config.mailserver.fqdn}";
      $config['smtp_user'] = "%u";
      $config['smtp_pass'] = "%p";
    '';
  };

  security.acme = {
    acceptTerms = true;
    defaults.email = "[email protected]";
  };

    firewall = {
      enable = true;
      allowedTCPPorts = [
        25 587 143 993 110 995 # Email
        80 # Nginx
      ];
    };

I'm logging in through roundcube, which works as expected. However, when I get to the point of composing an email to somebody, it just starts a "Sending message..." loop without actually sending anything.

I've forwarded the aforementioned ports on my router, yet it fails.