Please take what is written here with a grain of salt this is only my current understanding and would like it to be verified by someone who really knows what going on.??

With that said the scope of this document is to provide a background on how dual delivery works because even after reading the help articles the concept may be hard to wrap your head around.? mydomain.com is used as an example.

Pointers/Tips/Things I got hung up on and then learned.

The receiving of mail is somewhat passive; the receiving mail server inthis scenario mail1.mydomain.com just waits and hangs out till aconversation is initiated by a sending server.
In most cases the receiving mail server is oblivious to MX records it just knows it supposed to handle mail for mydomain.com, I believe that this is mostly true for single domain email server - any isp want to chime in here -
In scenario 2 there are no mx records that point to your personal mail server
In scenario 2 mail is set up in google apps to be routed to mail1.mydomain.com
References to "your personal email server" mean the server you were using for mail.?

The DNS records are as follows:?

www.mydomain.com A 10.11.12.5 ? <-- this record is just for illustration purposes to show what a a record to a web server looks like

mydomain.com MX 1 mail1.mydomain.com

An email is created and sent on a desktop in a far away land addressed to john.doe@mydomain.com, from now on we will call that email "Edward".
Edward travels through the internet on port 25 to the sending email server we will call this server "Outbound".
Outbound looks at the email and sees who Edward is addressed to, in this case Edward is addressed to john.doe@mydomain.com.
Outbound then using DNS looks to see what computer handles mail for mydomain.com in this case the MX record points to mail1.mydomain.com.
Outbound then initiates a conversation with mail1.mydomain.com and delivers the message to mail1.mydomain.com's queue where it awaits for a user to pick it up.

If mail server could speak english this is how the conversation might go

Sending Server (after using DNS to resolve the MX records): Helo receiving server I see that you are set to receive mail for mydomain.com
Receiving Server: Yep I am I see here in my config files that i am
Sending Server: Ok here is a message
Receiving Server: Thanks, got it, BYE

The DNS records are as follows:

www.mydomain.com A 10.11.12.5

mail1.mydomain.com A 10.11.12.6 <-- Notice the addition of this A record this is important this is the record that google will use to deliver mail to your personal email server

mydomain.com MX 1 ASPMX.L.GOOGLE.COM. <-- notice the change of MX records

mydomain.com MX 5 ALT1.ASPMX.L.GOOGLE.COM.?

Now lets follow that same email:

Edward travels through the internet on port 25 to the sending email server we will call this server "Outbound".
Outbound looks at the email and sees who Edward is addressed to, in this case Edward is addressed to john.doe@mydomain.com.
Outbound then using DNS looks to see what computer handles mail formydomain.com in this case the MX record now points to 1 ASPMX.L.GOOGLE.COM.
Outbound then initiates a conversation with 1 ASPMX.L.GOOGLE.COM. anddelivers the message to 1 ASPMX.L.GOOGLE.COM. queue where it awaits fora user to pick it up.
Google then (this is where it gets cool) sees your google apps settings and notices that the email is to be routed to an additional server in this case mail1.mydomain.com
Google resolves the address mail1.mydomain.com to ip address 10.11.12.6 where your personal email server is on and patiently waiting for connections.
Google initiates a conversation with mail1.mydomain.com / 10.11.12.6 and delivers the message.

If mail server could speak English this is how the conversation might go

Google Server (after using DNS to resolve the A record for your personal email server that was set in google apps): Helo receiving server I'm pretty smart I checked an A record that I was configured to check and see that you should get this mail for mydomain.com you don't even know that there isn't an MX record for you.
Receiving Server: What whats and A record, whats an MX record Oh well who cares moving on, I can see that the message you have here is addressed to john.doe@mydomain.com and according to my config files i handle mail for mydomain.com so hand me the message.
Google Server: Ok here is a message
Receiving Server: Thanks, got it, BYE

dual delivery concepts

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