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We're about to move a client from a "regular" Courier Mail Server at another provider's server to Google Apps with us. So at the same time, we're switching their DNS records to "point" to our web server and Google's MX records.

How can we avoid e-mail getting lost in the transition? Should we shut down Courier beforehand so that it doesn't receive any e-mail until the DNS changes have propagated? What will happen to e-mail that is sent to Courier but not received?

Also, what about users who don't check their e-mail very often, how do we move their e-mail from Courier to Google Apps when they're on the same domain? Should we temporarily move Courier to another domain?

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To avoid writing it all out again, I'll refer you to an answer I wrote recently. You won't have to worry about part 4. - Google will do that for you - the rest you will. Google can also log in to your mail server over IMAP as each user and sync their mail. Try out a test user first before changing any DNS settings. –  Ladadadada Apr 5 '12 at 8:17

3 Answers 3

up vote 1 down vote accepted

It takes time for DNS records to propogate - so there will be a period for which some clients think Google is your MX while others may try to use the old MX. Unless you've got a very unusual configuration, your old MTA will continue to accept emails for your domain until you switch it off / reconfigure it.

While you could just turn off your server, it's not necessary. You should reduce the TTL on your DNS records in advance if they are greater than an hour. Despite what Lucas says, you shouldn't expect the DNS changes to propogate in less than around 9 hours regardless of how low you set the TTL.

Set up fetchmail to retrieve the mail from the old system and forward it to the new.

You shouldn't be retrieving email (pop/imap) using the mx record. You can continue to access the old server using it's IP address.

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Just curious, can you please elaborate on why adjusting TTL won't help propagation time? –  Mike B Apr 5 '12 at 15:01
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Because a large proportion of DNS servers ignore anything below about 3 hours. –  symcbean Apr 7 '12 at 22:20

First of all do this over a weekend and well before you change the DNS records put the time to live really low (5 minutes).

Mail that's not received will either be dropped after a few hours (message undeliverable) , however mailservers will keep trying for a while.

I think the best thing you can do is first allow google apps to accept your email. Then you won't receive any more messages on the original mailserver (at this point you can shut it down) and you can start migrating.

I suggest taking a look at the special Google API on how to migrate emails.

Your last question isn't really clear to me.

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48 hours prior to the move add the Google MX record with a weight of 20 (assuming Courier mail has a weight of 10) to DNS. On the day of the move stop allowing smtp connections to Courier Mail. Incoming mail will not connect on the Courier Mail and will attempt the Google MX record since it's the next one on the list.

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NB. You cannot guarantee that your priority 10 MX will always be contacted when up (temporary glitches in the network, etc) so if you do this you need to ensure that Google first accepts your email addresses, otherwise they will reject and the emails will be binned. Then you would still need to make sure that those emails get sent onwards until switchover. –  jrg Mar 22 '13 at 14:34

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