Yeah, I noticed. It's a great way to make an extra $3 a month huh? Why don't you just offer it for free, if it's so important. Doesn't cost you anything.Even I offer my clients Offsite DNS for reseller plans.
They're going to miss out on it anyway until the server comes back online. They aren't magically going to get the email. Personally, I'd much rather have the email bounce, so at least the sender knows that something is wrong, rather than have it sit there for 4 or 5 days without a reply. When it bounces, people might even actually go check the website, and see that it's down. Then they know there's a problem. If it's a server outage, the client isn't going to be getting the email anyway until everything is restored on a new server.Email servers are bouncing email because It can't find the server. Now your client just missed out on important email
Me personally? We spread DNS servers around all over the place, but it really doesn't matter. If a server goes down and we can't bring it back up in 10-15 minutes, we've already started migrating clients to a temporary server in another datacenter. If a server outage or datacenter outage occurs, it's time to start moving clients immediately, not worrying about failover DNS.