The propagation rate for DNS is beyond anyone's control: TTL is advisory, not mandatory. But SMTP has MX records, which implement preference. If you have two SM

The propagation rate for DNS is beyond anyone's control: TTL is advisory, not mandatory. But SMTP has MX records, which implement preference. If you have two SMTP receivers on different networks, it's unlikely that both of them are changing IPs at the same time. If one of those is a paid-for VM, it's probably static, so it will not change IP barring business reasons. $5/month gets you a reasonable VM from any of many different service.