I'm hoping someone can help me with the following query.
I have a concern regarding changing the MX records on the external DNS servers to prepare for the exchange 2013 migration in coexistence environment which has exchange 2007 on windows 2003 R2 servers 64 bit.
We currently have exchange 2007 environment which is setup with an external DNS MX record of e.g. mail.contoso.com and mail.blogs.com both of which currently point to same external IP address e.g. 126.96.36.199. Both domains are registered to the company I work for. This is how it was setup when I first joined the company.
My concern is, would there be any mail flow issues, if I was to setup exchange in coexistence environment and configure exchange 2013 environment to use mail.blogs.com and point it to a different external IP address i.e. 188.8.131.52 (using exchange 2013 edge transport server in DMZ to send and receive emails) whilst leaving the exchange 2007 to use mail.contos.com pointing to the existing IP 184.108.40.206 (using client access and hub transport roles and not the exchange 2013 edge transport server). Our default SMTP email address is firstname.lastname@example.org.
Both domains contoso.com and blogs.com have been setup as accepted domains on exchange 2007.
Typically when you introduce Exchange 2013 into 2007 you want the Newest version of exchange (2013 in this example) to be the primary transport between the Edge Server and internal Exchange. So really all you need to do is NAT your current IP (MX) to the new Exchange 2013 server and all mail will flow into Exchange 2013 and Client Access will also be presented from Exchange 2013 and redirected to 2007 if required.
Based on what you are trying to accomplish I would think you would be trying to create an Exchange Resource Forest and then have them completely separeate and mail will route to each environment respecitively.
You will also need to make sure that your certificates are imported and enabled/configured on your Exchange 2013 environment as well.
Another thing to mention is that if you want to send from a different external IP then you need to make sure that you have an SPF record in place with the other IP added. If you do not do this your Reverse (PTR) record for your MX record will not match up and you will get flagged as spam on the net.
A good reference and high level steps I would recommend using Exchange Server Deployment Assistant.