Outlook Web Access (OWA) is a service that is provided both from your Exchange Server and from your Internet Information Server (IIS). So while it may seem that a problem with users getting into their mailboxes through OWA has to do with Exchange, it's entirely possible that IIS is the culprit. How can you tell? Microsoft has a document on its Web site that talk about various troubleshooting procedures in OWA. This document suggests that if you're having trouble getting access through OWA, you should create a new virtual directory in IIS and then put a new test document into that directory. If you or your user can open that document, then the problem you're having is not going to be a problem with IIS. You can then move on to trying to figure out what's wrong with the OWA service.
If, on the other hand, you cannot access the document, then it's likely the problem does reside in IIS, and you'll need to look into permissions and other settings in that program.
To find out, set up a virtual directory in your IIS server. The document linked above has detailed directions on how to do that. Then, make sure that you have provided read access to this directory, but clear all other access, just to make sure that others aren't going to get into your new directory and muck things up.
Once you've done that, then you can set up a test file for you or your user to access. You can easily do that in Notepad, just by entering some simple HTML code, such as:
This is a test file
Then, save this file in your new virtual directory with a .htm extension, calling it whatever you want. When you try to access the file with a browser, you should see a blank page with the words, "This is a test file" on the first line. No access, again, means that IIS settings are giving you trouble, and you should check out settings there. Access means your user is indeed having trouble with OWA, and you need to go there to solve the problem.
David Gabel has been testing and writing about computers for more than 25 years.