OWA not working for child domain

We're running Exchange 2003 Enterprise.  We currently have two domains, a parent and a child.  All users' email addresses are in the same format:  firstname.lastname@parent domain.com.  OWA has worked for us since the Exchange 2003 install until the last week or so.  Now I find that users who authenticate to the child domain cannot access their email via OWA.  Users who authenticate to the parent domain have no problems.

Any ideas?
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Are the users internal or external?

Have you tried to logon with the full user (UPN) name in the fomat  firstname.lastname@parent domain.com (as shown in AD profile)

The authentication defaults changed a little bit in EX 2003, maybe IIS add the wrong domain, if defined in IIS
lshellenAuthor Commented:
The problem occurs both internally and externally.

The users log in to two different domains.  One group logs in to the parent domain and the second into the child domain.  Also, to complicate matters, their user ids in AD are in the form <first initial first name><last name>@whichever domain.com.  Some users have tried the format firstname.lastname@parent domain.com without success.

I'll check IIS.
IIS is responsible for authentication, so the settings there will determine, which types on logon are accepted.

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