Hmm, that is rather confusing.  You can tell an engineer wrote it.  :)

I think the key is here:

5. Identify the legacy mail server as an inbound mail gateway.

Adding the legacy server as an inbound mail gateway lets the Google Apps 
mail servers know to expect a high volume of messages forwarded from it.

I think the "dual delivery" comes from the fact you're setting the mx 
record to naturally resolve external forward to your domain, and also 
that you will be forwarding from another mx server, your current one, as 
well. You're defeating their spam/spoof protection basically, for a 
duration.  You just let them know that someone will spike a detection 
algorithm and not to blacklist them.

I migrated a domain of mine to it to play with, but rather a dormant 
one.  If you're getting email today, this is a graceful transition 
method while you drainstop off your current mx.

-mb



On 11/17/2011 06:57 PM, Eric Cope wrote:
> Hey all,
> I am thinking about using Google Apps for my business email account. I
> read here:
> http://www.google.com/support/a/bin/answer.py?answer=178332
> about dual delivery, but their docs are a little confusing. Has anyone
> configured this? I am not sure what other details you might need, so let
> me know.
> Thanks,
> Eric
>
>
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