Working with internal and external domains in a single user object

I hope this is in the right place…

I have an internal domain “int.tld” and an external domain “ext.tld”.

I wanted my users to exist as “user@int.tld” with a proxyAddress of “user@ext.tld”.

I found the “Advanced Settings” under the user objects and tried setting “Alternative e-mail address” to the external domain, but nothing changed for the user object. I would have thought doing this would populate the proxyAddress attribute with this alternate address.

I also tried setting proxyAddress manually with the UMC preferences at the bottom, with no positive results.

For reference, I’m trying to set this up for use with a Proofpoint Protection Server (aka Secure E-Mail Gateway). This kind of integration is integral to the smooth operation of mail filtering.

My current case only involves my personal setup, but, as UCS becomes more mainstream, having this knowledge will be very important to all parties involved.