Exchange certificates and services setup for internal and external clients access on separate domains.

Question

I have the following on my local network.
Server DomainA -> Small Business server 2003/Exchange 2003
Server DomainB -> Windows 2008 R2/Exchange 2013
Clients Domain A ->  Windows XP/Outlook 2003
Clients Domain B -> Windows 7/Outlook 2007/2010
Problem:  I want clients from DomainA to log into Exchange on DomainB on the same local network.
I need to know how to setup the DNS on both domains and the certificates on the DomainB Exchange server
to accept the connection from the PC on domainA.   All connections from clients on domainB to server on domainB
work correctly but when adding accounts to Outlook 2003/2007 on domainA clients I am getting certificate errors.
I have purchased certificates for mail.domainb.com and autodiscover.domainb.com but I dont know how to get 
the clients on domainA to recognize those external URL's of the exchange server (with the certificates bound to them) from the internal network. Hence I get domain errors.
I am getting issues when a client on DomainA tries to add an Outlook mail profile to connect to the Exchange on DomainB
Any suggestions on how to set this up?
thanks

Answer 1

Domain A & Domain B are two separate AD Forests?
Users in Domain A either need mailbox-enabled user accounts that are in DomainB or a linked mailbox in Domain B to utilise the Exchange Server in DomainB. In either case with the help of the autodiscover service user can use the services in ExchangeB. 
If the client machines are member of domainA and you are trying to access ExchangeB you will then need to leverage a custom XML file for autodiscover and force the Outlook client to use this file. 
<?xml version="1.0" encoding="utf-8"?> 
<Autodiscover xmlns="http://schemas.microsoft.com/exchange/autodiscover/responseschema/2006"> 
  <Response xmlns="http://schemas.microsoft.com/exchange/autodiscover/outlook/responseschema/2006a"> 
    <Account> 
      <AccountType>email</AccountType> 
      <Action>redirectUrl</Action> 
      <RedirectUrl>https://autodiscover.domain.com/autodiscover/autodiscover.xml</RedirectUrl> 
    </Account> 
  </Response> 
</Autodiscover>
Then you need to configure the client machine to query that XML file by adding the following registry key:
Refer to XML file
for Outlook 2007:
HKCU\Software\Microsoft\Office\12.0\Outlook\Autodiscover
for Outlook 2010:
HKCU\Software\Microsoft\Office\14.0\Outlook\Autodiscover
STRING_value <your_namespace> = path to XML file
you can find more information in the following link.
Controlling Outlook Autodiscover behavior
http://blogs.technet.com/b/kristinw/archive/2013/04/19/controlling-outlook-autodiscover-behavior.aspx
CK