Have anyone tried this

- You put HostingCo A DNS as pri and sec
- Then you put HostingCo B DNS as third and forth
- By default all mail will go through HostingCo A's mail server
- In situation where HostingCo A mail server is down you can quickly switch to make HostingCo B DNS as pri and sec.

In this way, you can claim that your mail server is up for 365days. or at least a contingency plan to provide continuous services to your client.

Does it work?

It works but not the way we think it works! ;)

The email RFC states that it is the responsibility of the sending server to keep trying to deliver [for 72 hours normally] and give up only if the remote host says the mailbox does not exist or quota is full or something like that.

So for you not to lose any email addressed to you, it is not necessary that you should have secondary MX record, but it is necessary that your MX record resolves to a valid IP or hostname [even if that server is down at the time of delivery]. So redundant DNS with a remote secondary and tertiary nameservers is highly desirable.

That said, it is also desirable to have a secondary MX which will receive your mail as a temporary retriever and storage and keep trying to deliver to the primary until it comes up.

To answer your question shortly, it is possible to have a mail server set up where you will never lose any legitimate email addressed to you.

But it may not be possible [without spending a fortune] to have a mail server that will always be available and where you can check your mails once a minute :)

SMTP is not so mission critical because you can use your ISP's SMTP if your mail server is down.

This is my understanding , but I am just a fool! :D

In the DNS, your MX (mail exchange) record has a "priority" number just for this purpose. There's nothing fancy you have to do with your DNS except add more MX records - the lower priority entries are the first tried.

Then just set up one or maybe even two more shared hosting accounts on different networks, all set to answer mail for your domain(s). You will then have a very very reliable mail service.

And for that matter - go ahead and use the DNS of at least two of the providers you choose so that part is truly geographically redundant as well.

Piece of cake. Good luck.

- You put HostingCo A DNS as pri and sec
- Then you put HostingCo B DNS as third and forth
- By default all mail will go through HostingCo A's mail server
- In situation where HostingCo A mail server is down you can quickly switch to make HostingCo B DNS as pri and sec.

This is not how dns works, the different name servers are used for load balancing and not redundancy.

"This is not how dns works, the different name servers are used for load balancing and not redundancy."

What, are you serious? I'm sorry but I do not believe this is the case. In fact, standard Bind DNS has absolutely no load balancing capability whatsoever. Now I think that something like the UltraDNS may have some load balancing, but standard DNS does not. The different name servers are strictly for redundancy and have nothing to do with load balancing.

If resolvers don't know your address, they ask the TLD. It offers any one of its DNS's without regard to which is primary. If one or more of the servers crashes, it is better to have the redundancy of multiple "secondaries".
I use www.granitecanyon.com and www.secondary.com to add slave servers that are truly redundant.

and zoneedit.com i think :)

What, are you serious? I'm sorry but I do not believe this is the case. In fact, standard Bind DNS has absolutely no load balancing capability whatsoever. Now I think that something like the UltraDNS may have some load balancing, but standard DNS does not. The different name servers are strictly for redundancy and have nothing to do with load balancing.

This is what i use to think as well. Im no expert, but since isps started using bind9, web-sites will not work all the time if all the name servers listed under a domain are running ok.