Todd,

The only ways I can think of getting Autodiscovery to work in a split domain scenario is basically having two different DNS resolvers for the domain or using two different domains (one setup as a Domain Alias in SmarterMail).

For example, if the domain is split by organization and those users who are internal to the organization are using Exchange while those users who are external to the Exchange organization use SmarterMail then you could have your internal DNS resolve either an autodiscover A record (as this will be checked first by Outlook when setting up an account for Exchange) or have a different _autodiscover SRV record set internally than what is resolved by external public DNS. Either way you would then make sure that public DNS resolves the _autodiscover SRV record to your SmarterMail server address if only those outside the organization are using SmarterMail..

If there isn't a clear delineation of who uses Exchange and who uses SmarterMail in the same organization then you would need a secondary domain that is setup as a Domain Alias in SmarterMail and is aslo set in the IIS Host Headers along with the primary domain. You could then have the Exchange users use the _autodiscover SRV for the primary domain (example1.com) and the SmarterMail users would use the _autodiscover SRV for the alias domain (example2.com). The SmarterMail users would initially enter their email address as username@example2.com in Outlook instead of username@example1.com used by the Exchange users.