In news Larry W typed:
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,
email=lwapnitsky@***********

Oops - don't put real e-mail addresses in newsgroup posts, or in your news
account settings. It's a surefire way to get spam and other junk sent to you
(including viruses).

We have no problems accessing multiple users' mailboxes by entering
the server URL with the users' email (minus domain),

You mean, the alias.

e.g.
https://servername/exchange/lwapnitsky

Sure, if you have to access a mailbox you don't 'own', that works.
Otherwise, when you log in, you get *your* mailbox.

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.

What do you mean by a "program that needs to access those mailboxes" ? What
software is it? I'd say this is an issue with the software, not with
Exchange, really - have you contacted the software developers?