HTTP error when sharing briefcase/documents externally

[Ian Forbes]

I've installed ZCS in a local domain (i.e. zcs@corp.domain.com). I've created another domain that my users reside in (i.e. @domain.com). When I share out my calandar or briefcase or documents to external users, they get an email address with the internal domain (i.e. http://zcs.corp.domain.com/home/...).

How do I go about changing that url that external users will receive to (i.e. http://mail.domain.com/home/..) which resolves publicly? I put mail.domain.com in the 'Public Service Host Name' in the admin console - domains section. When I share out my briefcase externally, the user does get this url but when they click on it they get:

HTTP ERROR: 403
permission denied: you do not have sufficient permissions
RequestURI=/service/home//Briefcase

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I'm not sure if it makes a difference but mail.domain.com is only resolvable from the outside.