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We are currently attempting to move away from using a "local" mail(exchange) server to an cloud based offering for all our automated emails. The problem is that we send and receive thousands for emails a day and its uptime is quite critical so the business do not want to put all their eggs in one basket, so if we would like to use a cloud based offering(mailgun) they would like a backup if this goes down. So my question is:

Would it be possible to set multpile A, TXT and CNAME records to multiple IP address so if one mail server goes down we can automatically start sending emails from the fallover(without them being blocked doing a reverse DNS lookup)? I know we will still need to adjust the MX record for incoming emails but that is acceptable to not receive emails for a short(1-2 hours) of time.

Does this make sense?

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Are you sure your provider doesn't already provide geographic redundancy and queuing? –  SpacemanSpiff Jul 6 '12 at 18:18
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3 Answers 3

so if one mail server goes down we can automatically start sending emails from the fallover

You can't (easily) have machines in different locations with the same IP address. You don't need an 'A' record to send email from a server. You cannot implement failover by changing DNS records.

The important stuff is to have any server which sends emails listed in your SPF TXT records (if you actually have a published SPF).

If you are talking about a MTA gateway machine, (SMTP messages originate on other machines and are all routed through this server before accessing the rest of the world) then that's a different matter - you can influence this using DNS but not by waiting for an outage to change the records. SMTP is designed to accomodate such scenario by publising multiple DNS MX records with different priorities, Clients should attempt to connect to each server in the zone ordered by the MX priority - if it fails to connect, it should then try the next. in practice most MUAs won't bother looking further than the primary MX - but if the client is an MTA, then IME, these all behave correctly. You haven't told us how your outgoing emails get to the MTA - so it's difficult to advise whether this will work in practice.

Sorry, but you clearly have no understanding of DNS and little understanding of SMTP. You need to do a lot more reading than just looking at the answers here.

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+1 for all of it and especially that last paragraph. –  John Gardeniers Jul 7 '12 at 1:13
    
That is so rude, I cant beleive the arrogance of some people –  user127496 Jul 7 '12 at 20:14
    
Perhaps instead of being so aggressive and rude it might be better to suggest some reading or perhaps a blog post... If this is how the community acts towards people that are less knowledgeable then it is doomed to fail. –  user127496 Jul 7 '12 at 20:24
    
You think that's bad ... –  Iain Jul 7 '12 at 20:26
    
@user127496 It's arrogant to point out that you're misunderstanding very important technology? To point out that the consequences of that misunderstanding could be devastating to your business? You should be thankful that someone had the huevos to tell you now before you found out the hard way. –  Wesley Jul 7 '12 at 21:04
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So your issue is twofold:

  1. How do I send/allow mail outbound from multiple servers in case one fails?
  2. How do I have the ability to have failover between mailservers for inbound email in case one fails?

Regarding 1:

There's nothing that prevents you from having multiple servers that send email out. Right now the main method to show that your servers aren't spam is using SPF (Sender Policy Framework) Records. All you need to do is make sure both sets of outbound servers are listed in your SPF record and you should be good to go (presuming, of course, your internal systems that generate the emails are configured to talk to both sets of mailservers). In addition, if you have multiple MX records, the reverse DNS lookup issue is somewhat obviated.

Regarding 2:

You use multiple MX records:

10   CLOUD.MAIL.SERVER
20   LOCAL.MAIL.SERVER

or whatever. If one fails to respond, the sender will try the other, and you will have no loss of mail. The tricky bit here for you is how your internal systems/users switch between one or the other. You can do things like set up multiple Outlook profiles, or multiple email accounts within the same Outlook profile, etc. for users. If your automated send systems are also receiving email, you need to make sure you know the procedure to have them check each set of mailservers, or how to switch between the two at need.

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This is basically what a secondary or backup MX does. You can basically setup either a local or off-site MX box that is able to queue mail until the primary comes back online and then will deliver all of the mail to the primary.

This way no mail is bounced back to your senders. Either way you would need the primary mailbox servers to come back online to actually route and deliver the mail though.

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