Exchange 2003 with multiple Front End servers

One of my customer is having one Head Office and a Branch. Each location will have one Exchange 2003 Back-End and one Front-End. I want to keep exchange servers in 2 different Routing Groups as physically there are connected through WAN links. OWA site will be published on the ISA 2004 server which will be acting as internal firewall.
To make both sites as failover site for each other I would like do the following things:
#Create MX record (10) of Front-End01 pointing to in Head Office. (Primary)
#Create MX record (20) of Front-End02 pointing to in Branch Office. (Secondary)
If the mails cannot be delivered to via Front-End01, then the mails automatically delivered to Front-End02.

Q1) I would like to know, under what circumstances and conditions the mails are sent to Front-End02 (secondary) instead of Front-End01 (primary)?

Q2) Do I need to setup the OWA on both the locations and publish on the ISA 2004 on both the locations?

Q3) What settings should I do on the Back-End servers to deliver the mails internally between routing groups?

Q4) Any other suggestion based on the above scenario to make complete redundancy?

I am badly in need of your suggestions.

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If one server is completely unavailable then internet servers should start using the second MX record for email delivery. That is the point of multiple MX records.

For your two URLs, that is up to you. If you have both servers reacting to the same DNS name then you will need to configure both in DNS and have round robin DNS in place. That will not give you redundancy, because the DNS system doesn't know when one server is down. You would have to look at some kind of load balancing technology to sort out which server is live or not.

Otherwise two URLs will make it easy for your users to know which server they are connecting to.

When you come to purchase SSL certificates, do not be tempted by wild card certificates, as those are not compatible with Windows Mobile devices.

The scenario that you have outlined above will only give you redundancy for inbound email traffic - not for the mailboxes themselves.

The MX record behaviour has nothing to do with Exchange. Therefore if you read up on how MX records work, you will get your answer.
Basically if the lowest value MX record is not available or cannot be reached, then the higher value record is used. The exact reasons for when the higher value is used differs between servers.
The exception to that is when spammers come along. They will usually use the higher value record in the (often correct) belief that the second MX doesn't have the same level of spam protection as the primary.

OWA will be on both frontend machines whether you want it to be or not. What is your choice though is whether OWA can be seen from the internet. That is where your firewall or ISA servers come in. (Yes I did say servers, because if you want the redundancy then you need to have two of those as well - one at each office).

You will need a routing group connector to get email delivered between the routing groups. Also make sure that you have not put a smart host on to the SMTP VS of any Exchange server, as that will disrupt the message flow.
If you want emails to go out through the Frontend servers, then create an SMTP Connector and set the default SMTP VS on the server you want to use for outbound email as the bridgehead.

exp_eeAuthor Commented:
Thanks Simon for your detail answer.
Shall I consider that if I shutdown the Front-End01 which is pointing to MX 10, then the mails will be diverted to Front-End02 which is pointing to MX 20?

If I publish OWA of Front-End01 to ISA01 and Front-End02 to ISA02, the url address should be the same i.e or I have to select a different name for the secondary something like ?

exp_eeAuthor Commented:
Convinced. Thanks for your answers.
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