nixos/modules/mail.nix

{ ... }:
{
  systemd.tmpfiles.rules = [
    "f /run/agenix/gandalf_mail 0777 payas users -"
  ];

  programs.msmtp = {
    enable = true;
    setSendmail = true;
    accounts.default = {
      auth = true;
      host = "smtp.purelymail.com";
      port = 587;
      tls = true;
      user = "gandalf@bhankas.org";
      passwordeval = "cat /run/agenix/gandalf_mail";
    };
  };
}