Questions about mxrecords (24 hours propagation period)

Good day,

If I change my MX record, i have to wait 24 hours approx before the DNDS cache is updated around the world...

. If someone tries to send me an email, while my mxrecord is still pointing to my old adress, what happens to the email?  Will it bounce?
What  if the  a certain mail server Qqeue timeout is set to like 12 hours before sending an NDR?


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will bounce back, ndr will only work if email gets to emails server
Barry GillChief of StaffCommented:
if you are aware the change is coming, set your domain TTL down to 5 minutes or 10 minutes. That way when you make the change there is a very small window of old dns cache.

The old address, is the server still listening there?
what will it do if it receives a mail for you?
Probably put it into a mailbox in which case there will be no bounce or NDR or even failure event.
If it is off, most mail servers are configured to retry for 3 to 4 days. Unfortunatelty, not all of them will re-check mx records when they retry, so those messages will take 3 to 4 days to fail in queue on the senders server

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Rich WeisslerProfessional Troublemaker^h^h^h^h^hshooterCommented:
Assuming your MX record has a 24 hour expiration, per your SOA:

Foreign server X has a cached copy of your MX record at time = -1 second (one second before you made a change), and successfully delivered that mail.  At time = 0, you make the change.  If that server than receives another mail to deliver to you, and it uses the cached MX record, presumably it won't be able to deliver the mail to your mail server.  (Assumption: NO server responds on the cached address on port 25.)  If that server is set to attempt delivery for 12 hours before sending a NDR, the NDR will be sent in 12 hours.

If the foreign server received a cached copy of your MX record 13 hours before you made your change, the message might well be delivered sometime after 11 hours.

You can change the Time to Live (expiration) on your SOA or an individual record in the period leading up to a change to avoid this sort of thing though...

If you are using a Windows DNS server, (at least in my 2003 console), there is a setting while under advance view, to change the TTL for an individual record...
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maxalarieAuthor Commented:

You saying t hat  If i change my TTL to 20 mins, there will be only 20 mins of downtime? I am planning to edith their Mx Record  later this week.

I have 2 exchange servers, both are up. They are on different domains. (we acquired a small company)
I want to change their MXRecord  to my mail server.

I have created the user  accounts on my domain already, the email policy  will be created so  my domain will accept mails from their domain. (I added an smtp adress on each  user from the new branch in my domain)

maxalarieAuthor Commented:
Last question,

I should change only their TTl right?  Is there a need to change mine as well?
Rich WeisslerProfessional Troublemaker^h^h^h^h^hshooterCommented:
Yes... although technically there shouldn't be any actual downtime.. hopefully just a few messages that are delayed a little while in delivery.  (And barrulus actually beat me to the suggestion by a minute.  :-) )  If the TTL is currently 24 hours, on the records you are planning to change, 24 hours before the change, start cutting back on the TTL... and make certain the decreasing TTLs are on the DNS server that remote servers would be using to resolve the address (and not just name servers used internal to the network.)  
Rich WeisslerProfessional Troublemaker^h^h^h^h^hshooterCommented:
> I should change only their TTl right?  Is there a need to change mine as well?

You should only need to change the TTL now on the record(s) you are planning to change later in the week.
And after the change, don't forget to set the TTL back to a 'normal' setting again to take the load back off the name server (although in most cases, the load should still be fairly light).
maxalarieAuthor Commented:
As of now, They have a WebHosting Company with mxrecords, nameservers and everything.  Should i change the TTL there as well?

maxalarieAuthor Commented:
I have checked my settings on my 2 exchange servers. Both are setup to 1 hour. However my Bluehost  TTL is set to 14400 seconds and i cant change it to a lower value. Is that normal?

Both office have a Web Hosting contract and there  are  redirections made to  their respective  corporate IP for emails...
Rich WeisslerProfessional Troublemaker^h^h^h^h^hshooterCommented:
If you go to and enter the domain in which you are making the change.  At the bottom of that record will be name servers -- that is critical place where you need to make certain the change is made.  (I suspect/assume that will the Webhosting Company you mention, but that's how to confirm where...)
Chris DentPowerShell DeveloperCommented:

It will queue for a time, then bounce eventually, normally within a day.

Modifying the TTLs is a good approach, but not as safe as adding the new address to the MX prior to the change-over.

That is, if you were to add a new Host (A) record pointing at the new IP address (new mail server name) prior to the change, and add a secondary MX pointing to that name you would incur no downtime regardless of propagation.

Barry GillChief of StaffCommented:
The approach of adding a new MX host will only work if the old server is going to be decomissioned at move time. If it is still listening on port 25 then having the dual MX's is going to create confusion with multiple delivery locations.

As you own both of these servers, you have the ability to shut access to port 25 at cutover, so that would help, but then you have to go back to remove the old entry.

If your users are moving at the same time then you will have downtime for them anyway, so 10 mins downtime is fine.

If the servers are at the same location even better because you can reditrect traffic for server 1 to server 2 and then your MX records will ahve no affect at all. I assume they are not in the same IP ramges.
maxalarieAuthor Commented:
Last Question..

Does exmerge works with office 2007?  Does it  export contacts, address books etc as well?
Chris DentPowerShell DeveloperCommented:

ExMerge is more of a server-side tool, only the version of Exchange matters. Typically you would use ExMerge for Exchange 2000 / 2003, and Export-Mailbox (Management Shell) for 2007 / 2010.

Don't forget that ExMerge has a 2Gb limit on PST files (old file format), worth bearing in mind if you have anyone with a particularly large mailbox. None of Outlook 2003 / 2007 / 2010 and Export-Mailbox suffer from that limitation (Outlook 2000 does).

Otherwise both tools export everything in the mailbox tree, Contacts, Calendar, Notes, etc, etc. If you're doing a full system move (one AD to another AD), I strongly urge you to export the LegacyExchangeDN values from the current domain, then import them as X500 addresses in the new domain. Only applies for cross-forest (different Exchange org) moves but without that you will have problems with replies to mails sent prior to the migration, and problems with ownership of calendar items.

maxalarieAuthor Commented:
I have sucessfully rediorected the emails.  Here is what I did..

A couple days before the redirection, (minimum of 24 hours) i have changed the TTL  of  DNS management tool. i have changed it to 300 seconds. By default it is set at 24 hours most of the time.

48 hours later, I logged back in the dns management tool, and i changed the Mx Record to point to ip hostname:

After that,   I have created an email policy in  Exchange system Manager, an i accepted the domain for all the new users account that i had already created on Active Direcotry.  (there was 8 new users)

Every thinig works perfect now.

Thanks to everyone. Its really apreciated.
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