My organization is currently using SBS 2003 running Exchange. Our naming
convention is such that the Windows login names are the first name of the
user, and e-mail addresses are the first initial/last name (in most cases).

Example: Larry Wapnitsky - login=larry,

We have no problems accessing multiple users' mailboxes by entering the
server URL with the users' email (minus domain), e.g.
https://servername/exchange/lwapnitsky

The problem lies in that we have one program that needs to access these
mailboxes that is unable to. The base URL information is entered
(https://servername/exchange), then the mailbox is selected via Active
Directory. The error message received is "The mailbox selected cannot be
accessed. Please check your credentials."

This is strange, as the same administrative user is running this program as
can access the multiple mailboxes. Please advise.