We've got a Windows 2003 Server Active Directory environment. In AD you can enter a person's email address. Back when we used to have Exchange, when creating a new user it would also create an entry in Exchange. We've not have Exchange in years, but when creating accounts, or modifying their profile in AD, I've left the email address portion blank, because I wasn't sure what would happen. Now I'd like to know, can we create an account or modify an existing account, and also add their email address, even if it isn't a part of our domain? (Such as when they have a Hotmail or Gmail account.)
If you don't have Exchange or any other AD-integrated mail system, that attribute is effectively unused; it can be used to store information, like the user's actual email address, but nothing will make any use of it and you will be able to see what it contains only by doing an AD query.
So, yes, you can safely store the user's email address in there; but it will not be available to, say, the Outlook address book, because there is nothing server-side actually using it; it will only be useful if you open the ADUC console (or any other tool which queries AD) and look for it.
By the way, even if you had Exchange, that attribute is only written by Exchange, never read (Exchange stores the user's actual addresses in a completely different place); so Exchange will initially put there the user's primary e-mail address, but you can overwrite it with anything you want, without any consequence at all.