I would like to implement the following :
Scope Exchange 2010 SP1 RO6
Same domain + 3 AD sites + 3 mail GW in each site DMZ
Il would like that internal mails sent from, lets say, site A to site B going through the mail gateway in Dmz of site A. Then mail GW site A talk to mail Gw of site B then sent to Hub in site B.
I known that this configuration is "very special" and I do not have the reason why my client wants to do that.
Thanks to advice if this is possible and how to implement.
As you probably know the Exchange 2010 routes mail based on active directory site links. You can influence it a bit via a special exchange routing costs but that's all. (Ref. Understanding Message Routing).
However, we've also had customers inquiring about such a setup in the past. They didn't want to have their expensive internal bandwidth consumed by e-mail. Technically it should be possible (no hands-on experience myself) by creatively configuring your network routing and/or using WAN optimization devices (which are also expensive) to redirect SMTP traffic outside. That being said it's definitely not a scenario supported by Microsoft. Imagine you need to open a support call with MS one day or another.
Furthermore there's an aspect of confidentiality; do they really want to sent all of their internal mail over the public internet? (it's not clear from your post whether you'd want to connect both of the gateways via either VPN or extranet.)
My recommendation would be to talk to your client, ask him about his motivation and write a case where you document why you advise against this strategy.
Thanks for your help.
You confirmed my first thought.
I will definitely follow your recommendation and talk to the client about this.
Il would like that internal mails sent from, lets say, site A to site B
going through the mail gateway in Dmz of site A. Then mail GW site A
talk to mail Gw of site B then sent to Hub in site B.