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MX Record Change Strategy and Failback Time?


Our MX record was setup at the wrong office years ago. We have a route setup to port the e-mail to our Exchange Server at the corporate office. I'm going to add a new MX record at our main office where the Exchange server is located. I spoke with the company hosting our ISP and with the company who makes our e-mail filter. They suggest the setup below. What do you think?

Setup new MX Record called Mail1 at corporate office with priority of 3. Current MX record at remote office has priority of 4.

In the slim chance I've made a mistake and this doesn't work, how long does it take to failover back to the old MX with priority 4?

If there was an issue with Priority 3 MX, i fix it, then how long does it take to fail back from Priority 4 MX?

I spoke to the company hosting our MX record. They told me all of this was instant. I find that hard to believe and look forward to your input.
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4 Solutions
Tony GiangrecoCommented:
MX record changes can be instant if they have a lightning fast system and they send immediate updartes to the zine servers, but that is normally not the case. I agree with your assumption that norml MX recrd changes take longer. I've experienced 4 to 6 hours to be normal. In some instances it took 24 hours based on the ISP and zone server speed.
Yes, a failback is instant - a sending server tries step be step, beginning with the smalles priority (usually 10) to the highest, until it find a responding server.

What you got stuck with is the MX-record itself: If you set it's lifetime to 86400 (which it usually has) it might be cached for 24hours: If you made a mistake, it will prevent your company to receive some mails for the next 24hrs.
Change the TTL's of your MX records to something low before the change, like 300.  If the current TTL is say 86400, then change to 300 and wait 24 hours to add the new MX record.  After that you can change the MX records back to something higher if you desire.

Just make sure you do a telnet test externally to your office ip on port 25 and that you get your server.  If that works you should be good to go.

But as the above guys said, drop the TTL to as low as you can 300  like Papertrip says gives 5mins TTL, give it 24 hours then add the A record with your site ip address. mail1. Check you can telnet mail1.domain.com 25 and you get your exchange server. then add it as an MX record with higher priority 3 as your ISP suggested.

Mail will then try mail1 (priority 3) if this delivers correctly your solution is working fine. If mail1 doesnt work then the sending mail server will try the next in line mail ( priority 4) and will deliver via that server. As long as you put in the right ip address and you take care to make sure the TTL's are sufficiently low you shouldnt have too many problems.
First LastAuthor Commented:
I thank everyone. Here is the offical plan:

1) Change TTL from 24 hrs to 5 minutes. Request to ISP.
2) Setup routing/firewall for IP of what will be my new MX record. Only port 25 needed.
3) Verify telnet to port 25 on ip of new MX record shows welcome message from Exchange server.
4) Verify TTL changed to 5 minutes via mxmailbox.com
5) Add new IP to MX as mail1.domain.com with a priority lower than the current MX record. Request to ISP.
6) wait... sleep... watch leaves fall...
7) Verify the change went through. Mail should be coming in new MX.

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