Secondary MX records

Hello,

I am trying to get a better understanding of how MX records work. I have only ever setup exchange servers with one MX record. If I added a secondary MX record will mail get sent to that additional server too? What I am trying to get is something that will still recieve mail if the primary MX is not reachable....and once it comes back up again I need something where secondary MX will then re-send all the mail the primary server missed. Is this possible?
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Dan560Asked:
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MesthaConnect With a Mentor Commented:
I have to disagree with the above post about "normally" MX records being pointing to an ISP. I wouldn't say that was normal at all.

In theory, MX records should work where all email is delivered to the address with the lowest cost. The server with the highest cost only gets email when the lowest cost is not available.

In practise, that doesn't happen. I find that email will go to all servers in the MX records. I am running a long term test involving my own MX records and that of one of my clients. We both have multiple Internet connections and multiple SMTP gateways. I am finding that the split is roughly 60/40 in favour of the lower cost MX record. IE, 40% of email comes in via the higher cost server.
I am also finding that about 80% of spam traffic is coming in via the higher cost record because spammers specifically target them. The theory with that idea is that the higher cost has a lower level of spam protection because the email administrator doesn't think it will get the same traffic.

I am not a fan of backup MX services because of the limitations in their service.
The DNS Made easy service above gives a classic reason why - they cannot defend against a directory harvest attack.

What I prefer to do is use a dynamic DNS account to give me control over where the email goes constantly.
http://www.sembee.co.uk/archive/2007/02/02/38.aspx

-M
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flyingskyConnect With a Mentor Commented:
You can set up a secondary MX record, just give it a higher reference number than your primary one. But I don't believe you can setup it to automatic re-send emails once the primary one is up.
have a look at here, especially the "Multiple Mail server" part
http://bobcares.com/index.php/blog/?p=29
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Syedm2Connect With a Mentor Commented:
Normally, a secondary MX record is configured and pointed to the ISP.
So,if the primary MX pointing to ur mail server is down,it would failover to the secondary MX record and ur isp will keep ur mails till ur server is up.
then,once it is up they will send all emails to u.
So,it works as fault tolerance so u dont loose any emails.
MX priority is what determines the primary and secondary MX.
Have a look at this:
http://www.dnsmadeeasy.com/s0306/prod/mailbk.html
http://linuxgazette.net/issue36/tag/80.html
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Syedm2Commented:
I disagree to ur post as well
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Dan560Author Commented:
Thanks, so there is no way the secondary MX server can resend the mail to primary MX when it comes backup?
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MesthaCommented:
You have to look at how the email gets passed from one host to the next.
Where I have put in place relay servers they have been exactly that - relay servers. All they do is queue the email and wait for the original server to come online. You cannot get access to the messages.
To get access to the messages you either need a queue viewer (which will only show you the raw messages) or you have to deliver the messages to a mailbox. Once the messages are in the mailbox it isn't possible to forward them on to the Exchange server without doing each one individually.

Most backup MX type systems are simply queueing servers, with a queue viewer functionality bolted on top. Most ISPs will not give you the viewing option, so you would have to wait for the email to come down.

-M
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